The female body is beautifully arousing; double that on a tv screen...Yum! I'm a woman and I love seeing two women together, so long as the acting isn't atrocious.
There is something so gracefully sexual about two women together, it's difficult to define, and for certain, stimulating in a wholly different manner than a man/woman sequence.
And I agree with gigantor, there are some nasty male porn stars out there, too! I'd much rather watch two women than see some guy who looks like he was plucked straight off the set of the Jerry Springer Show.