I'm not exactly sure what your point was, but I'm going to take a stab at it, sorry if I didn't get it substantially right.
I believe that the nature of the sexual urge is programmed into us, to varying degrees, and at a time when sex and reproduction were NOT connected (animals don't connect them, if you believe in evolved humans, there must have been a time when we didn't connect them), sex was just engaged in because it was what someone (probably usually male) wanted to do.
When it was discovered that not only were they related (which, like I said before, was probably when animals were domesticated and penned up, and could be separated from each other) but that there was a paternal line of inheritance, as well as a maternal line, those facts became the reason for controlling sexuality. We finally have relatively fool-proof means of separating the two. Now, the sexual urge is subject to other considerations, not the least of which is finding a suitable partner that adequately meets one's other needs.
Being a slutty groupie is not going to gain anyone very much more than accidental pregnancy, disease, and a feeling of being used, but between that extreme, and the extreme of shutting off one's sexual expression 100%, there are some happy mediums that do NOT involve making babies inside of a marriage.
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".