Are there indeed! I didn’t know that, thanks. Hmm.
Well, I dunno. If people want to try to shift the meanings of words, or use words differently from others, okay. But someone who’s doing that is presumably not a believer in linguistic prescriptivism. So unless those people are also highly inconsistent, they presumably wouldn’t try to say that there is only one valid definition of ‘bisexual’.
Prescriptivists generally rely on dictionaries, or at the very least on common usage, to distinguish their one ‘correct’ definition from all the other ‘incorrect’ ones. Someone whose definition is not in dictionaries and is not supported by common usage really hasn’t got any basis for promoting their definition over anyone else’s. (I mean, they can argue that it has greater merits and ought to be adopted, but they can’t argue that others are wrong.)
Since we’re on this subject, I actually do think there’s a kernal of binarism lurking in the word ‘bisexual’. It isn’t as simple as ‘but etymology!’. Etymology, as mikroblogolas says, is not a linguistic top trump. But I think the difficulty comes from the tension between the word’s etymology and its meaning. It’s important that this is a very obvious and well-known etymology. A lot of people who would never guess the original connotations of (to use Melinda’s examples) ‘vocabulary’ or ‘rape’ will immediately, without even thinking about it, grasp that ‘bisexual’, when used in the same way as ‘heterosexual’ and ‘homosexual’, must mean ‘attracted to precisely two genders’.
So if we have a definition, in ordinary usage, of ‘bisexual’ as ‘attracted to women and men’, and we put that together with a very hard-to-ignore etymology of ‘attracted to two genders’, we end up with a word that really only makes sense if there are only two genders and they are male and female. If that doesn’t make the word in itself binarist, it makes it a word that’s much more comfortable in the gender binary than out of it. It’s transparently a word that a non-binarist world would not have come up with. That doesn’t mean it can’t be repurposed, but it does make me wonder why it isn’t better just to use words without these problems.