Venomsnake mentioned slash specifically -- yaoi is much more mainstream than slash, especially in Asia.
If you want to talk about more mainstream material, particularly what appears at the checkout counter, one confounding factor is that written material in general is less restricted and stigmatized than visual material. There is no ESRB or movie ratings board for books, and parents who would never allow their children to watch an extremely violent movie or a porno will let them read absolutely anything at all. So written pornography can be put out at the checkout counter in a way that visual pornography can't. Women who want visual porn get it online or on late-night TV.
