Im really confused about your argument. Law ≠ morality. Slavery was also okay Im every country for so so so many years, would somebody who said thats had at the time be considered mentally ill to you? Also same sex marriage was illegal in most places until RECENTLY. Would you consider that a mentally ill thing to want when it was illegal? It’s literally the same argument - Nobody is forcing you to partake, nobody is telling you it’s the only way to love. It’s not hurting anybody - Theres literally extensive talks between partners to see what is and is not considered bad between them, so I don’t see the problem. Threesomes also happen between 3 consenting adults, why are you so bothered about something you’ll never be forced to do that makes somebody else happy?