But it's different when it's involving a heroine. It's no longer just another run-of-the-mill hero/villain pairing, it's a flat-out abusive relationship. Which would be fair enough if this were concerning hero/villain pairings regardless of gender as a whole, but it seems to exclusively apply to ones involving women. And this isn't even referring to pairings that have sexual abuse in canon--these are ones that, were the girl changed into a boy, would be seen as pretty standard hero-villain dynamics.
Take a pairing like Kylo Ren/Rey from Star Wars, for instance. It involves the heroine getting captured and incapacitated by the villain, the villain forcefully trying to extract information from her mind, the heroine ultimately overcoming him via her own mental prowess, and it all eventually culminating in a life-or-death duel that, again, ends in the heroine overpowering him. It's a very, very standard hero-villain dynamic, yet because it involves a woman, it's not regarded as such. It's seen as an abusive relationship, nothing more, nothing less.
This isn't limited to het pairings, either. I've been neck-deep in controversy for shipping Jasper/Lapis from Steven Universe for similar reasons, to say nothing of the variety of other "good" and "bad" gem pairings that get similar flak, so I know it can't be.
It's a little disturbing to me, because it reads as though no matter how strong the heroine, no matter how she overcomes the villain on her own, no matter how non-sexualized it is, she's going to be the designated victim the moment she's facing off against them.
Maybe it's just the circles I've been in--I could be missing out on some hero/villain discourse involving male heroes. But I've had no problem looking up content for, say, Apocalypse/Charles from X-Men without having to run into "But it's abusive and wrong!" even though it's a pairing that involves an ample amount of kidnapping and mind rape too (far, far more than Kylo Ren/Rey ever had).
I'd also go into fandom's weird fixation on comparing any form of peril a female character gets into to rape, but I think I'd mostly be repeating what I've already been saying.