Because it's typically far more exploitative of the participants, it's nearly impossible to prove when it isn't, and due the competitive nature of brain plasticity in most mammalian lifeforms, it demonstrably rewires the brain's arousal pathways, eventually leading to a desensitization and disconnect from real-world sources of arousal.

[Why yes, Brainy did research a hell of a lot of sex-related science when he finally got a boyfriend.]