Whether any consensual sexual activity between adults has harmful effects on society as a whole is very much open to question. That's what we have legislatures for.
Since there is no way, no how, that I would find a naked sweaty hairy man the least bit attractive, I guess that is why I am flummoxed by the amount of passion involved here.
Hitting yourself on the head with a hammer is extremely damaging to yourself. You could get injured, bleed, require medical care, or even die. It might cause brain damage, or disgust people who see you with the claws sticking out of your scalp. Obviously we must set out rules against it. Perhaps I won't hit myself with a hammer, but since it is so tempting to others, it needs to be enforced so that it can not be done. Well, ok, it might still be done by those hammering themselves addicts, but still, it serves a societal good. Hitting oneself on the head with a hammer is bad, and it is in our best interest to prevent people from doing so.