My point was trying to define what is nature, and what is not. You kept saying that human nature changes, yet you never gave any good example. In fact, you kept pointing at things having nothing to do with basic human instinct.
I would also point out, both for this post and others you've been posting, that the pleasure comes TO facilitate the need/want. I've already posted this as has Aquinasfan, but you seem to want to think that pleasure came after utility. Not so.
You don't think animals have pleasure eating and having sex? Oh they do. Why would your dog prefer real beef over his processed kibble? Why would any "entire" animal try to have sex during cycles? It's not "simply instinct". It's the awareness that it would be pleasurable that's planted in there. The animals wouldn't do this stuff if it wasn't "fun".
I do indeed acknowledge that pleasure has long been a part of it, although I cannot imagine that most sub-primate mammals have anywhere near what the primates do. In any case, animals are not bound by ethics, only instincts, and whatever limitations we humans can put on their interactions.
That's just the same sort of "limits" that religion puts on human sexual behavior. When sex had an extremely high chance of leading to pregnancy, and paternity could be questioned, rules for who could have sex with who made sense from a societal point of view, and religion's enforcing of these rules, through civil law, had some justification. When any two people having whatever with each other is NOT going to result in offspring or disease, it's nobody's business any more.
I know you and Aquinasfan have some sort of need to find justification for the old rules, but I believe in a hundred years from now, people are going to read this on some archive, and wonder what the fuss was all about.