Pain isn't abnormal. He's a dom with stronger pheromones than Butal, so he dominated and had sex with Brutal. That's normal in vampire society of this book--the strong dominate in every way, including sexually. What abnormal in this society is being strong and choosing or enjoying a submissive role. This society values strength and choosing submission is like wanting to be a "loser".
We know homosexuality isn't a problem because the vampires can have same-sex marriages and there are plenty of open homosexual scenes. We know perfume boy (Shell's ex fiance whose name I temporarily forgot) was a sub--and that's what made him abnormal. Shell wasn't at all at risk for being gay--he's "shameful" secret is that he's a sub.
Oh man, how beautiful shell is.
It looks like the history is repeating itself. Since shell has Pain's blood in his veins, they both are "abnormal"
Ugh, even in a fiction, fantasy manga... they could have called it something like, divergent. WHY "abnormal"?! ┗( T﹏T )┛