I'm not sure. Magnus Bane is the bisexual vampire demon guy right? According to a fan fiction website Archive Of Our Own they're a more popular couple than the main couple.Girls can't imagine themselves as bisexual half demon men so they can't mentally masturbate to it. She would have been better off writing "bella swan girl meets bisexual half demon man and they have a flirtation before finally fucking 5 books later".
Slash fan fiction and yaoi seems especially popular with teen girls and young women. I'm not sure if they'd willingly buy it in public particularly in more conservative regions. It's not exclusively porn, but much of it is porn. I think the massive piles of it laying around the internet speaks to some unexploited market. Bella only exists for girls to project themselves onto anyway, what if there was two Edwards.
It was a dark and stormy night when Sasuke Sephiroth Malfoy locked eyes with Blast Hardcheese.