It doesn't seem to be offending anybody else, either, and I think that's part of its appeal. In the story I heard on NPR about the controversy, or more accurately the "controversy," scare quotes intended, the teens interviewed seemed unable to muster any enthusiasm for the thesis that there is something daring about the lyric. Part of what a high school kid isn't going to say on NPR, I'm guessing, is that "girls kissing" may come right between "beer pong" and "drunken sing-along" in the stylized Kabuki drama known as "frat party." Though I really should ask someone 20 years younger than me to confirm, I surmise that the song isn't popular because it breaks some taboo (unlike Jill Sobule's song of the same name and sentiment 13 years ago, which got itself widely banned), but rather because it describes something that actually has some currency -- heterosexual girls kissing each other with the permission of their boyfriends for minor and brief titillating effect. It's a song about the slightest act of consequence-free rebellion possible. The lyric might as well be "I got a small butterfly tattoo on my hip" or "my boyfriend is half-Jewish." But again, I don't expect a summer song to tax my brain or challenge my politics, so nothing wrong with that.
Beyond that, I agree with commenter Tortoiseshelly (