The only sin more unforgivable than writing sympathetically about a character who hates the reader's object of lust (like Cassandra) is to write a female character who is loved *by* the object of lust. I find slash fans can be incredibly hateful to any character who has a canonical attraction to a popular male lead.

A good thing, too, that neither Avon nor Blake had an ongoing het love interest in the show. (I wonder whether Anna Grant would have fared fare differently in fanfic if she had been around longer...) However, more recently I've been revising my opinion as to whether this is a slash-related phenomenon, because of the explosion of Harry/Hermione-'shippers related fury after the publication of HBP...