Sexual Relationships 
Sexual Health 
It's a question women frequently ask and wonder about: Why do guys look? Why ogle girls on the street or naked women on the Internet? What do they get out of it? Even for a guy, this behaviour can sometimes be difficult to explain. For my first ever Take, I've chosen to write on this issue, given that it does cause friction and misunderstandings in daily life. (According to the Daily Mail, men's tendency to check out other women in swimsuits on the beach is the #1 cause of arguments for couples on vacation.)
What follows is my attempt to explain this phenomenon based on science and some conjecture. It's not intended to judge this behaviour as right or wrong. Any article like this must necessarily deal in generalizations about the genders, so not everything will hold true for each individual out there. It will also go into some explicit detail regarding the sexual aspects as well, so reader discretion is advised.
Classic sculpture, "Three Graces"
MATING INSTINCTS
At the most basic level, everything in nature wants to survive and reproduce. So humans, like our animal cousins, have evolved to look for and respond to certain attraction cues in the opposite sex which aid in that mission. The attraction cues one gender responds to can be very different from those valued by the other. Since females have only a limited period of fertility and the risk of producing birth defects increases with age, the cues men respond to are actually markers of youth and vitality - things like good skin, lustrous hair, big clear eyes, facial symmetry, and a slim figure. A flat stomach indicates an empty (ie. available) womb. Ample breasts signal the ability to feed babies. Curvy hips differentiate her from prepubescent girls who have more of a boyish figure and are not yet capable of giving birth.
Seeing enough of these qualities together lights up the reward centre of a guy's brain, releasing feel-good chemicals, urging him to look further. Similar reactions in humans can be triggered by things like drugs, money or chocolate. A Harvard study found that beautiful female faces in particular trigger the reward factor in males and a desire for more of that view. If you show male test subjects a series of male and female faces, they will linger longest on the attractive female ones. Ever walked into a store or bakery, noticed a pleasant aroma and, without even thinking, took a deeper breath to take in more of that scent? Guys may be this way when it comes to sights of the female body. So if a man hangs a pinup or swimsuit calendar on his wall, it's partly because he receives a small hit of pleasure or reward every time he glances at it.
Elvgren pinup, c. 1950
Evolution has endowed the male brain with robust processes for mate selection which are always running in the background. When an attractive girl in tight or revealing clothes comes into view, she will seem to jump out from the landscape and grab his attention. This is nature's way of saying, "Wake up! Pay attention to this! This is important!", so as not to overlook a potential reproductive opportunity. This momentary hijacking of the male brain happens automatically, whether he is single or taken. We see it when a guy turns his head slowly to watch a passing female jogger or has wandering eyes when talking to someone in a low-cut blouse. In summertime, long, slender legs can draw a man's eyes like a magnet.
This attention does not mean he desires her as a person or would even consider leaving his current relationship.
But if he is at least aware of her presence in the environment, then his male instinct has done its job. Such mental stimulation might occur several times in a day, but becoming physically aroused by brief random encounters is rare. Usually once the attractive stimulus is out of sight, she quickly passes out of mind and is forgotten.
Most men know better than to openly stare in public. But controlling this urge is more difficult than females realize. It means giving up the pleasure chemicals that naturally would be released by a longer look. When this becomes a daily effort, absent any other outlet or source of relief, it can feel frustrating. It's no fun being constantly pushed by one's biology to look but never being fully able to, and increases curiosity for that which remains unseen. Sooner or later, most guys will seek a harmless way to look freely upon that which they're programmed to like most: the female form.
A BRIEF HISTORY OF LOOKING
Before modern times, this need was serviced by a variety of live entertainments such as burlesque shows, peep shows, striptease, pole dancing and beauty contests. But a turning point came in the 1800s with the invention of photography. This meant the human body's appearance could be exactly replicated on paper quickly and easily. Not long after, another technology called halftone printing allowed nudes to be widely distributed in magazine form for the first time. The first such periodicals came from France in the guise of art or naturism journals and became popular with men. Adult titles went mainstream in the 20th century, becoming available in almost every variety store and bookstore. They are consistently amongst the top-selling magazines of any type even today, despite competition from the Internet, according to Wikipedia.
French erotic postcard, c. 1900
Images of women can catch a man's eye and make him look as effectively as girls on the street can, eliciting the same reward response. But since pictures are inanimate, a guy can stare and fully indulge his instinct to look – temporarily letting it off the leash in a relatively safe way. Glamour models usually display an idealized female shape with abundant fertility cues that maximize the male pleasure response.
The power of the visual to excite men has no analogue in women." - Dennis Prager
SEX DIFFERENCES
It is difficult for women to conceive of being turned on solely by fit bodies since female sexuality does not appear to operate this way. One recent study by Dr. Meredith Chivers of CAMH showed videos of athletic nude men walking down the beach or doing yoga to female test subjects as the women's arousal was monitored. The male bodies excited them about as much as the viewing of landscapes. By contrast, a guy can simply look at a nude female model in a neutral pose and feel a rush of interest. This difference may have evolved from the fact that male arousal is absolutely mandatory for sexual intercourse while female arousal technically isn't.
A Finnish study in 2011 also discovered that our brains process nude human bodies more efficiently than clothed ones. A naked body elicits more neural response than do objects, clothed people, or even closeups of faces. Male brains proved most adept at processing nude female bodies, prioritizing these forms over all others. According to the researchers, “Such a perceptual ‘highway’ for processing of sexual cues is highly beneficial for triggering sexual behavior, and subsequently ensuring mating and reproduction.”
THE NAKED TRUTH
Recent discoveries at the University of Florida indicate that our species began wearing clothes some 170,000 years ago to migrate to colder climates. “It’s interesting to think humans were able to survive in Africa for hundreds of thousands of years without clothing and without body hair,” said lead researcher David Reed. This means our species has been naked for far longer than it has worn clothes. In such an environment, female nudity would have been commonplace, and it was this environment that shaped male mate selection over many thousands of years. In modern times, however, public nudity is practically non-existent. This has made it somewhat taboo and more alluring to a male brain adapted to a very different environment.
Some feminists see the proliferation of scantily-clad and nude women in men's entertainment as a conscious effort to keep women subordinate and subservient to men. However, the simple fact is that physical traits like skin quality and waist-to-hip ratio are easiest to evaluate if the female is naked. That way, every aspect of physical appearance can be seen and evaluated at once. Such evaluations are what the male brain was built for. Researchers Ogi Ogas and Sai Gaddam, who do experiments on sexual desire, have said that, “men’s brains scrutinize the details of arousing visuals with the kind of concentration jewelers apply to the cut of a diamond”.
Elvgren pinup, c. 1950
In our culture, women's legs and upper part of breasts get the attention they do largely because those are the areas left uncovered by Western fashions. If customs were different and women's midriffs or buttocks were on display, guys would probably fixate on those areas instead. In Victorian times when the entire body was covered, women's hair was considered highly alluring. Novels from the period often describe a female character's long, flowing locks (another marker of health) at length through flowery, romantic prose. A glimpse of her ankle would be highly exciting and definitely draw a man's gaze.
However, individual guys can differ quite a bit in what they find attractive.
Most men favour some variation of young-ish, healthy and curvy, but within these limits a wide range of preferences still exists. Thus, one guy's idea of perfection may be “a large booty and massive rack”, while another may be turned off by that look and feel zero attraction from seeing it.
BOYHOOD
Shapely bodies hold no fascination for prepubescent males. But once adolescent neural circuits are laid down during the brain's biggest growth spurt since infancy, boys start noticing girls in a new way. This includes girls' bodies. Prior to the Internet, fashion ads and marketing materials aimed at a female audience – especially swimwear and lingerie catalogs – were many boys' first experience with the idealized female shape. That's assuming they didn't already have access to an older relative's men's magazines. Certain newspapers also featured a “Page 3 girl” who was scantily clad or topless.
As these materials found their way into homes through the mail, male children going through adolescence would start to notice and take an interest in them. As boys grew older, most would quietly start acquiring their own swimsuit issues or calendars, and eventually graduate to magazines showing full nudity. This was such a common and seemingly normal progression that many adults saw it almost as a rite of passage. Thus the idea that “all guys look” was born.
Women's fashion catalog, c. 2000
Male interest in such images likely peaks in young adulthood – the teens and 20s. This is the period when the male body is literally surging with hormones and sex drive is highest. It also happens to be the time in a male's life when he is most likely to be single. In ancient times, the onset of puberty was more closely aligned with the average marrying age. Boys and girls would marry in their teens and become sexually active right away. Today, due to our comparatively long lifespans and years spent in higher education, the average marrying age has reached a historic high of 29 for US men (32 in the UK). This means modern men live almost 20 years of their sexual prime before they marry.
During this period, male bodies are producing 20-50 times more testosterone than females their age...
...testosterone being the hormone that fuels sex drive for both genders. It sounds almost cliché to say "guys are visual", but the fact is their brains contain 25% more visual neurons than females have. Also, the area of the brain devoted to sex – located in the hypothalamus – is literally more than twice as large in guys, containing twice as many cells. This means guys are assailed by sexual thoughts several times daily, whereas girls average maybe one a day (this according to Dr. Louanne Brizendine, author of The Female Brain).
When male test subjects are shown a neutral scene of a guy and girl conversing, this sexual area of the brain lights up, suggesting that they interpret the scene as a potential sexual encounter. Females shown the same scene have no such reaction, seeing it simply as two people talking. All that extra real estate devoted to sex in the male brain would have driven our primitive male ancestors to restlessly seek out a variety of mates, seizing reproductive opportunities wherever they arose.
Dr. Brizendine, who has also authored a book on the male brain, claims that "Young teen boys are often totally, single-mindedly consumed with sexual fantasies, girls' body parts, and the need to masturbate. Their reluctance to talk to adults comes out of magical thinking that grown-ups will read between their spoken lines and the look in their eyes and know that the subject of sex has taken them over, mind, body and soul. A teenage boy feels alone in and ashamed by his thoughts ... he thinks he is the only one consumed by such intense sexual fantasies and the constant fear that someone will notice the erections over which he seems to have no control. Compelling masturbatory frenzies overwhelm him many times a day."
A typical "adult" magazine, c. 1988
SELF-PLEASURING
The way that boys interact with nude images in private is frankly sexual and thus rarely discussed openly. Looking closely at a model with ALL her attraction cues on display (ie. wearing little to nothing) can flood a boy's brain with testosterone, dopamine and adrenaline, similar to a drug high. Now the process of arousal begins: the heart quickens, breathing deepens, pupils dilate, and the penis erects. In younger guys especially, these reactions can happen instantly. (Male animals can react this quickly too. Observations of stallions -- uncastrated male horses -- show that when one sees an attractive mare, he exhibits immediate interest, followed by erection in under two minutes.)
As the penis grows and stiffens, its nerve endings become so sensitive that the lightest touch sends signals to the brain which are experienced as pleasure. Without thinking, a boy will start to rub or stroke his erection as his eyes wander over the female's body and face, focusing on parts he likes most. (Some studies that track male eye movements as they look at sexy images have found that women's faces attract their attention the most!) This forms the basic process by which female images are consumed by straight males. The beautiful shapes processed visually by the brain and the tactile sensations from the penis reinforce each other for a highly pleasurable and erotic experience. Is a guy in this state necessarily imagining sex with the model or entertaining other fantasies about her? In many cases, no. The female form itself can be so stimulating to guys that no extra fantasizing is needed to maintain erection.
(Masturbation is not endemic to humans. For example, stallions spontaneously get hard around 18 times a day, with about 75% of those erections leading to "masturbation", ie. repeatedly smacking the penis against their stomachs, usually without orgasm. Researchers McDonnell, Henry and Bristoif believed these incidents to be "normal behaviours which occur with greater frequency and regularity than previously reported.")
Gradually, a guy's focus of attention will narrow, centering on the image, until all other thoughts and distractions are blocked out.
This level of concentration is necessary to achieve orgasm, and aroused males can attain it easily. Additionally, the logic centre in the brain's frontal lobes shuts down, allowing the pleasure centre to take over. This is important, as it suggests a lack of conscious or rational thought taking place. It means concerns over whether a guy might be comparing the naked woman to his real-life partner or to other girls he knows are irrelevant. In this state there is barely any thinking going on at all, it is more of an entranced feeling state. The male will simply drift off or "zone out", basking in the potent stew of brain chemicals released. A female poster on another advice site commented that “According to a male source close to me, what goes on in his head is: 'She's hot. She's amazing.' Then the thinking logical part of the brain shuts down and there isn't much going on in the head.....”
This hypnotic state can last anywhere from a couple of minutes to hours, and because it mimics the sensations of sex, it usually ends in orgasm. Male orgasm, despite being relatively easy to achieve, can feel like an enormous exertion. It is accompanied by ejaculation – a series of 10-15 contractions in which pulses of semen (aka “cum”) forcefully "shoot" from the urethra. This event is experienced as a huge relief or a much-needed release of pressure. Because “the amount of arousal can also affect the volume of ejaculate” (goodinbed.com), guys may use pictures of girls not just to achieve arousal but also to increase the volume of semen expelled. Larger and longer ejaculations feel more satisfying and intense.
Some 250 million sperm are released when a boy shoots cum.
A male who has just ejaculated will be unable to do so again for a period of time. This is due to the post-orgasm brain releasing oxytocin and prolactin to inhibit further arousal and induce calm while the body immediately gets to work on replenishing its sperm stores. The guy quickly loses all interest in the imagery he has been studying, feels drowsy or sluggish, and his penis goes down, signalling an end to arousal. Sexual thoughts and interest seem to lessen or disappear while sperm stores are low. But once enough sperm has regenerated, over a period of hours or days, the male finds his sexual interest returning, along with renewed urges to look. The male body can produce over a billion sperm per week -- that's 1,500 every second! This incredibly high rate of production continues round the clock, 24/7. Thus, a pattern of tension buildup, followed by release, repeats itself over and over again.
This explains why single males often keep a collection or “stash” of beautiful images on hand that they can refer to as needed. Some guys regard their masturbation as nothing more than relieving base urges. Others insist it's also about admiration and appreciation of women and their art. Today, for better or worse, hardcore pornography has largely replaced nudes as masturbation material for many guys. But informal GAG polls suggest that over 80% of guys still look at nudes, occasionally or often. Wanting to see girls naked is a natural desire that all straight males feel to some extent, and practically all psychologists and sexologists today accept such looking as normal.
CONCLUSION
To summarize, guys are programmed to notice female beauty, feel strong urges to look, and are rewarded by pleasure reactions in the brain for doing so. With effort, they can avoid looking in an obvious way, but these passing glances are rarely enough to satisfy their natural curiosity. So most single males also turn to images of attractive women and girls. Teen boys get very hard erections from seeing the female body unclothed and virtually all will masturbate to it at some point. This is a universal experience of maleness and appears to happen naturally.
Many factors contribute to the male sex drive, from high testosterone levels to the huge size of the preoptic hypothalamus to round-the-clock sperm production. Because looking is so strongly and instantly rewarded, the behaviour is difficult to control. Probably most straight males reading this had to take a long look at the picture below before finishing this paragraph. If you did this yourself, you've helped to demonstrate how the male brain processes its environment, placing priority on female shapes. This shifting of attention is an unconscious process that happens in a split second, beyond a guy's control.
Most Helpful Guy