LOL. if you think about it, is all breasts are , are fat really?. why are we men turned on about it.? lol. I'm a guy and sitll can't seem to work it out:P lol . like I mean girls obviously aren't turned on by a penis so what's the go ? :P
Its an evolution trait. All mamal have sex in pretty much the same way. Doggy style, So naturally the male is attracted to the buttocks. These form the giant bulls eye to aim at when having sex.
Humans have developed more complix socialy and also the abiltiy to have sex front ways so the mammary glands became larger, more bottom like. So tits are really a bottom that you can look at while you talk to girl. Or in my case something to stare at to avoid eye contact.
Because men are programmed to find women attractive so we'll want to spend a lifetime with one and raise a family. In modern civilizations breast are kept mostly covered so anything that has do with exposing them in any way, shape or form is a turn on for most men. The same goes for legs, hips, butts, etc. A penis isn't one of things that turn on women because women have their own ideas of what turns them on about men.