Neil:
Which strikes deep into the real heart of the whole great slash debate. It's about sex. People (for whatever deranged and unfathomable reason) care more about that than they do when it comes to preferred styles of dancing. (I would personally say that Avon Doesn't Dance. But not because he *can't*, you understand...) Fans construct their own ideals of the characters, develop an emotional investment in them, and assign greater or lesser priority to each aspect of that ideal. Sexual
orientation
tends to be near the top of the priority list. Dance floor expertise tends not to. (Although we might imagine a hypothetical fan who might argue passionately that his or her - though probably her - Avon is a ballroom dancer and not give a squit about who he sleeps with, it's unlikely.)
Personally, I have a great deal more emotional investment in 'my' Avon's musical taste than his taste in sexual partners. I can believe him with (in alphabetical order) Blake, Cally, Jenna, Tarrant, Servalan, Vila, or for that matter, xyz sexworker at abc space-port, or being 100% faithful to Anna; but in all cases he likes Bach.
That's what it all comes down to - emotional investment. Fans talk about 'my Avon' or 'my Cally' in explicitly proprietorial ways.
Everyone I've ever heard use it does it to distinguish their personal character, the one they write about or draw, from the series (canonical) version.
Tavia