I hate to say it but broadly speaking, I agree with Graham
Our (that includes me) aversion to events under discussion is mostly down to taboos/tradition/religion or 'morals' (whatever the hell
they are).
Objectively, this boils down to two consenting adults, no matter how sick/twisted/depraved we find it.
.............and thats all I'm going to say about this
(probably said enough already)