I'm gonna talk about yuri and yaoi here, just in long-winded response to one of Mooncalf 's entries . There's a LOT of yaoi out there. Most of it is written by women. Most of the time it has supremely good art. Most of the time it's startlingly tasteful and well-done.
Tasteful porn is good.
Unfortunately most of the porn out there is tasteless and nasty. The difference, as far as I can tell, seems to be who's making it. Women tend to do a better job at being tasteful most of the time. So it tends to be tasteless and godawful. The quality isn't nearly as high as yaoi, for the most part, and as Moon said most of the yuri, good bad or ugly, is locked away in hentai pay sites.