Now, as a fairly straight chick I can understand the appeal, but I can't figure out why dykes would want to watch men instead of women. I know, I know, straight men do sleep with women, so it's less of a leap. But think about it in terms of what we classify as a perversion, using the term in a positive sense to describe something that seems provocative because it is outside the realm of your own experience and what you could ever actualize in your own boudoir. One reason some straight men get turned on by lesbianism is because it's a perversion to them, something in which they can never indulge. So it isn't inconceivable that gay women would get turned on by something that, from their perspective, is a deviation.
Deviation is fun.
We all like a little variety. No doubt there are some lesbians out there renting straight porn as well, aroused by the shenanigans of those wacky heterosexuals. I read some stuff on the Net about male circumcision that really freaked me out. It also says that women say that uncircumcised men seem to enjoy sex much more than circumcised men. This isn't to say you should be pissed at your parents.