There’s a bunch of reasons those types of guys are jealous.

The main thing is attention.

"Men go off on single moms because they're held up in society as some victim who didn't know what she was doing."

That quote helped me figure out why these guys hate single moms the most. Because Single moms get sympathy, praise, and accommodations. Things single childless men are far less likely to have. Also, since these guys see men as victims of society, single moms are proof of women being victims of society due to the (in)actions of men. That means feminists are right about something, and feminism is their enemy.

Another source of jealousy is that these guys are single. Meanwhile, even someone they see as beneath them, is doing better with love than they are. They whine will about it being a preference, but normal people dont dive into hate and mockery of people they dont prefer. It’s also why they call stepdads simp beta cucks. Also, mothers generally have the mutual love of their children, a type of love these guys most likely dont have themselves.

“What about women?”

It could be jealousy, but I think its insecurities and despising children. I will expand on that another time. Right now, I’m focusing on the main hate, which is from single men.