I wonder how much of this is rebellion. Boomers would have grown up in a world where overt sexism was acceptable. They differentiated themselves from their parents by being progressive.

Fifty years later, their grandkids are in a world where it's much harder to get away with sexist behaviour/talk. Now the situation is reversed: kids can differentiate themselves from their parents by being sexist edgelords.

I'm not saying that accounts for all, or even most, of these regressive attitudes. But it's probably a factor for some.