Lesbian Porn
Ok, so this is blantant Google-bait, but my treatise on Britney's breast isn't getting that many hits lately.
Anyway, I've heard various reasons why girl-on-girl sex scenes are so interesting to men. For example, some would argue that these excite men, because they seek the challenge of “converting“ the women, while gay porn threatens their sense of manhood.
I have a simplier theory: Count the Naked Woman.
Gay Porn: Zero naked woman - zero male interest.
Conventional (male/female) porn: One naked woman - significant male interest.
Lesbian Porn: Two naked woman - extreme male interest.
How difficult was that???