This kind of societal screwed-up-ness is exactly what's predicted by those who say that you shouldn't have sex outside of marriage. People get in the habit of hooking up, which sets up unrealistic expectations on both sides. We're losing the ability to have real relationships. Remember that article about how people in Japan "aren't having sex anymore"? The HN comment consensus was, they are, they just don't have lasting relationships, because that's too hard.
People take the easy way out, even if it makes them unhappy later. If you can have sex without being married, that's easier than taking the time to develop a deep relationship, even if you're never really happy.
Of course, nothing will work if people don't have good character, which is really the problem. If people are getting married just to have sex, not because they're prepared to make the sacrifices required by living and working together, then things only get worse. We've instilled an entitlement mentality in lots of people, and we're reaping the results.
Anyway, if you wanted to know why premarital sex is considered immoral in most major religions (the big 3 monotheistic ones, anyway), you're watching it, live.
Young women generally will only date men her age or older. So for example a 22 yo woman would date men in the large pool between 22 and 30. Meanwhile the 22 yo man doesn't have much opportunity to date older or younger. However, when the woman hits 30, the situation has reversed and the 'marriageable' men in her pool are dating younger women.
Whether someone is a '5' or an '8' isn't necessarily an inherit characteristic and has a lot to do with their life circumstances.
This means, when women are in their early twenties, they're use to being in relationships or hooking up with the most attractive men. Although I hate ratings, let's say men 8-10, are getting together with all the women 5-10. This can't work in the long run, because people partner up 1-1, so those 8-10 men end up married to 8-10 women. What happens with all these women 5-7? They spent their twenties with men 8-10, and now they're left with their equals, men 5-7, so they feel like they're settling, or stepping down. At the same time, men feel like they're dating up, because they never received attention from these women before. Meanwhile, they're equal, but prior history leaves this skewed perspective.
In short, average women in their twenties get attention from great guys. As people begin to settle down, average women are only left with average men, and they're left wondering where all the good men have gone.