Sex researchers Baumeister and Tice wrote about sexual economics.

“A heterosexual community can be analyzed as a marketplace in which men seek to acquire sex from women by offering other resources in exchange.”

From an evolutionary standpoint it makes sense that women wouldn’t want a partner that can’t provide security for the couple when the woman would be vulnerable if pregnant/nursing.

Young men in particular have fewer resources of value to offer than at any time in most people’s lives. To that point, it’s not like young women are dating any better, so even if they are willing to be the sole provider, most are unable to do so.

With the traditional partnership which historically provided companionship out of the question, men are left yearning for female companionship.

Another point the researchers make, is that men will always yearn, while women have a generally easier time abstaining until conditions are right.