Because people try to cram all sexual ethics in the framework of consent, in which every bad sex act must necessarily violate consent in some way.
https://thingofthings.wordpress.com/2018/08/24/critique-of-just-love-part-one/
Consent is a crucial heuristic that should be taken seriously. But if you treat it as the single foundation from which all other things flow you get bad results.
The only people who try very rigid consent-based ethics outside sexuality are ancaps, I guess.