So, like, this seems like it should be the very most obvious and lukewarm of takes:

Incest porn started getting big around the same time that it started being totally commonplace, in normative middle-class society, for men and boys to live in the same house as step-mothers and step-sisters and step-daughters – women to whom they have no blood relation, but with whom they are expected to interact in exactly the same way that they would interact with their biological mothers and sisters and daughters.

I’m not even saying that I believe this, particularly.  I’m saying that I’ve seen a lot of public bafflement about the rise of incest porn, and I’ve never once seen anyone raise this point, and that fact surprises me.