But why? It's not like lesbians are a different kind of woman or something. They have all the same parts, from what I've heard.

What part of lesbianism is so appealing if it's not the woman?

If you're just a man who's interested in a woman, why complicate that?

In a word: "validation". Namely, of the AGP's fantasy that he's a woman.

If a bisexual (much less straight) woman is attracted to him, there's no "proof" that he's being perceived as a woman. Because neither is exclusively female-attracted. The former could be into his maleness; the latter would HAVE to be. (Well, really BOTH would have to be-- it's not as though bi women are gonna see him as anything other than male, after all-- but I mean in his mind.) So such women-- the only ones capable of finding him sexually desirable-- can't float his AGP boat.

Therefore, nothing other than a lesbian will do. Precisely because his being a man is a dealbreaker for her. Which is to say, the part of lesbianism that's so appealing isn't the woman herself... it's that female homosexuality makes her the ideal prop for his all-important "I'm-a-LAY-DEE" fetish.