Dumping on bisexuals doesn't even make that much sense, though; maybe it happens because too many homos have been shot down by bis.

Huh. That hasn't been my experience or the experience of other bisexuals I know (to the best of my knowledge) unless maybe by "shot down" you mean would not agree to dump and then forsake all those with men parts in order to date a lesbian. My experience has been that bisexuals are accused of being on the fence - not able to commit to being queer - or not queer enough. I've had gay folks bitch at me that I don't know what it's really like being queer because I can "pass" as heterosexual. Of course they've never been around for such delightful experiences as being accused of trying to convert a heterosexual woman into gayness - never mind the whole assumption of attraction thing that went on as well. I could go on but it's getting off the point now...