older men are watching porn and fantasizing about the same girls they did when they were in their 20s and they want a self-insert.

Well yeah, it goes both ways. However, I've been forced to see enough Tiktoks of these types of "daddy" type content to say women are watching it just as much, just in a different venue than raw porn.

step-parent

Plausible deniability I'd reckon. Avoids any legal issues in places where incest is illegal, gives you an out to pretend there isn't anything uncouth, and basically just a general weasel out of any uncomfortable questions.