(okay, maybe this is partial fanservice, since visual porn tends to be aimed at men and written porn tends to be aimed at women... though that doesn't explain it entirely, unless FMM porn — despite pushing boundries — still tends to be heterosexist)
(well, most writers/directors are heterosexual)

Or maybe it's more rooted in the male gaze thing. Or maybe the fact that some women honestly do work really hard to elicit pleasure in others. (wait — but really good-looking guys don't?)

Either way, women tend to be more aware of what's expected of them, since they have to be.

(see: marginalized people are keenly aware of the other group's privilege, but the privileged group has a very hard to seeing their own privilege)