Hypergamy: Great answer by Jerry.
In most cultures, the accepted norm is that women date and marry equally or up and men are expected to date equally or down. This of course goes hand in hand with the idea that men are supposed to be the protectors and providers. As Jerry mentioned there are some valid evolutionary and social reasons for hypergamy, but as more women go to college than men and as more women have professional careers, hypergamy simply can't be sustained as a norm.
You mention dating sites and that's a great example. If more women than men are now going to college, how can most women on dating sites, date men who are more educated, and of a higher socioeconomic status than they are? Yet this is still the accepted norm. People think it quit acceptable for a woman who's got a terrible job, in debt, has kids, has mental health issues, to marry an educated, successful man, but a man who wants to date women who are as educated and financially stable as he are considered selfish. It just doesn't add up anymore.