@branman65 @ImperialAgent @Saber I think it is trivial to accept that it is given that we know what is the natural intention of sex and the incentives it created with regard to our biological nature. It would be hard to have an exception for that and not beastiality.

(Ignoring Onan as I think it is very fair to say he had a lot of aggravating circumstances that were his more obvious errors)