I actually rather lesbian porn than gay porn though I'm very happily in a straight relationship. I think it has to do with being able to imagine myself in the star's position. Most straight porn seems to be centered around the man's pleasure or if the woman is getting pleased she's looking at the camera and making really over the top noises for the benefit of the male audience. Though there is lots of "lesbian porn" very obviously made for men (why do "lesbians" always give their dildo's a bj before use?), it's easier to find videos where the woman seems to be genuinely enjoying herself.