Women get to choose which men they have sex with. When they're young, in particular, they have lots of men begging them for sex. They can have sex with the ones they want and reject the ones they don't want.

But when it comes to giving out relationships and commitment, men hold all the cards. I don't have to give out commitment to any woman I don't want. While men are often thirsty for sex, it is women who are thirsty for relationships.

Women may all want the top 10, 20, 30% of men, but they can't all have them. 80% of women throughout history reproduced while only 40% of women did. That means a lot of women ended up as leftovers and abandoned single mothers, looking around for other men (not the biological fathers) to support them and their kids. Or they simply get old and dried up, and men didn't want to have sex with them anymore. That means it is hard for them to find commitment and relationships.

TL; DR- Men want sex more, women want relationships more. Women can deny sex, men can deny relationships.