Feel free to rate autistic, this ended up longer than I expected.
The shift from a community courtship society, where matches were made through community organizations like churches, schools, or youth clubs and actualized through a highly public courting period, was destroyed in the orgiastic casting-off of social norms during the Free Love generation. Now the sexual marketplace is a free-for-all with almost nobody entering relationships seeking long-term partnerships, and most people end up fucking before they even find out if they're intellectually or emotionally compatible. Not that people didn't have irresponsible sex before, but it was shunned then. Now society tells kids that irresponsible sex makes you a chad, or that it's "empowering," without preparing people for the emotional damage it causes.
What started out as "sexual liberation" actually ended up breaking down the ability of modern adults to form long-lasting relationships, as studies have shown that an increase in the number of sexual partners reduces a person's ability to pair-bond. In effect, the collapse of the dating marketplace caused by society ridding itself of traditional sexual mores and social organizations (and the decline in social trust and communities generally, which is a different problem altogether; see: Robert Putnam, E Pluribus Unum: Diversity and Community in the Twenty‐first Century) broke down the ability for people to find matches outside the remodeled dating scene. So the change in venue reinforced the change in approach, with venues becoming more sexually charged and approaches becoming more sexually charged. That's poisoned how people think about relationships.
What a lot of the women complain about on FDS is that they have unwanted sexual approaches, or that men sexually abused them. What a lot of incels complain about is their inability to act on sexual urges. Both of these arise from an insanely oversexed society where actually liking someone or wanting to start a family with them has vanished and the sexual marketplace seems like the only avenue for romantic fulfillment. Incels and Femcels are both ruined by the same system acting in different ways, one group shut out of it and the other chewed up by it, but both groups promised that the broken system is working fine.