You guys have made some good points. Actually I have a lesbian friend, and she really is a feminist, but her gf isn't, and they both have different opinions on this issue.
My friend's gf thinks it's the idea "1 naked woman == good, more than one naked woman = more good".
My friend, however, thinks it's because men want to think themselves as something so attracting that they can even make lesbians forget girls. She put it this way: "Two lesbians play together as long as the guy isn't there. But when the guy comes around, he attracts women so much that the lesbians immediately want him to join in. What a bullshit. Men are stupid if they think they can change us."
How do you see this statement? I don't believe any man could actually think that way (if you're heterosexual, you don't forget girls when you see a good-looking guy around, right?) but then again I can never be sure. ;D