in my mind this falls under bodily autonomy. people have the right to consent to anything being done to their body or not, but also that consent can be given or rescinded at any time. its unethical to legislate away our ability to consent to be shot or stabbed or bludgeoned or killed or whatever, and the person carrying that out shouldnt be punished at all for participating in consensual acts.
i think society in general just gets the ick from the idea of people consenting to things being done to their body that they personally dont understand or would never want and tries to stop it to satisfy their own feelings. we see this applied to all kinds of things like abortion, body modification, drug use, suicide, gender transition, sex acts, sexual partners, etc. its pretty patronizing and only causes additional harm to people seeking those things out without really doing anything to meaningfully stop them.
hopefully at some point in the future human culture will be able to internalize bodily autonomy as an inherent personal right. we're on the road but we've been hitting a few bumps lately...