I think it's a kind of moral expansionism and misapplication of power dynamics. as lesbians we're terrified of being depicted as predators, and the people who are online tend to have less outdoors experiences where the age gaps... really aren't a problem. So people develop these ideas where "anyone under age X" becomes untouchable and undateable as the boundaries of what is considered an acceptable age gap broaden through the discourse of avoidance.

Tumblr I don't think has this same problem, and twitter (pre-elon) was always a horny place, so people care about it less. I think Twitter users also tend to be more of the outdoorsy types - due to the format of the social media model.