Why is it a big deal for someone to be having sex out of marriage? I'm 19 and only have/had one sexual partner and don't see the problem.
Most Helpful Guy
It's only a big deal under pre-1960s moral codes.
For most of human history, your family/clan name, heraldry titles, lands, possessions, etc. were passed down in a defined way from father to first-born son (generally), and with a well-defined line of succession. As such, true-born sons were extremely important, both from a religious viewpoint and from a social/legal/economic one, and so having bastard children (children not fathered by the legally-married husband) was a big problem that had to be guarded against. Keep in mind too that religious views were much more closely integrated into social customs and mores than they are today.
Now, it was also recognized that teens are horny and want to have sex, and will. But pre-1960, sex was almost certain to lead to pregnancy (no effective birth control), so it was completely normal for people to get married at (what now seems like) a very young age - the average girl was married at 15, and being married as young as 12 and 13 wasn't unusual or especially notable. Girls were expected to be virgins on their wedding day, but this wasn't a very high hurdle given the young marriage ages.
All of this started to change post-WWII with the rise of science and technology, and came to a head as the Baby Boomers became adults in the mid-late 60s, fully embracing science and tech and rejecting the strict religion-influenced social mores (and had the tech and education to support them). Today, the average age of first marriage is 30, and no one in their right mind would expect girls to remain virgins til their 30s. But even though the times and morals have changed, people have found it hard to change what they tech their children - they tend to go back to what their parents taught them, even though they KNOW that those teachings are out-of-date. Too many people can't seem to think for themselves and make their own adjustments based on modern realities.1