I think maybe it's because so many gays are out. Lesbians actually have some level of admiration among young people. So when a slightly versatile woman looks around and does not see any men worthy of her favors, her gay and out friend doesn't look so bad.

In the past, gays were much more secretive, so they would not have appeared as possible sex partners for lonely young women. There really is hardly any stigma for being lesbian these days. But even back in the 60's a lot of people thought lesbians were hot.