Whatever it is, this article sure doesn't answer that question.
In fact, I've never gotten a real answer from any man I've asked that question. In that aforementioned article, the guy's answer seems to amount to "we're just wired that way." That doesn't help.
When a man is asked that question, what we--or, at least, I--want to know is...why is the thought of two women a turn-on? What are you thinking when you see it? Is it just "more for me," i.e. the thought that you can somehow be included and these women are not really off-limits to you? And that's interesting, if that's the answer, because I noticed how this guy's article wasn't truly about lesbians. Sometimes, I think men don't really believe there's such a thing. And, from what I've seen, when a man realizes a woman truly is a lesbian and doesn't have anything sexually to do with a man...he goes from turned-on to angry.
But the women he wrote about weren't that way. Angelina Jolie might have had a thing with a woman, but she's with Brad Pitt...and who can forget how utterly disgusting she was when she was with Billy Bob Thornton? They seemed to always be all over each other. And his female friends who openly admit women are attractive or that they would get it on with a woman...honestly, that's just how women are. He's right. Women are better able to openly admire other women than men are with men. I know plenty of heterosexual women who do, and I noticed the Angelina Jolie phenomenon (although I've never understood it) a long time ago. I also think there are more heterosexual women who are curious about same-sex sex than hetero men. However...that doesn't make these women lesbians or "so-called straight women." It's difficult to say what does make a woman a lesbian, but I think most women who really are lesbians would say those things don't make a woman one. Having sex with a woman once in your life doesn't make you a lesbian, and talking about maybe wanting to do that certainly doesn't.
And, as always, bisexuality is ignored. Sexuality is not an "either you are or you aren't" deal. Angelina could very well be bisexual with a preference for men. Or she could be, what I call, a "trysexual." A lot of these hit-it-and-quit-it girls--kind of what this guy's friends sound like they want to be--are trysexuals. Trysexuals have sex with people of their sex for the experience or for "fun," but they would never have a real relationship with or real feelings for someone of their sex. Or they might have the relationship, but it'd be, again, just for experience or "fun." They are the kind of chicks that real lesbians want to choke, because they play games with lesbian hearts and add to the societal notion that people "choose" to be gay.
I wonder, is there a real answer out there to the question of why men seem to be so fascinated with girl-girl action?