@ Roark First of all, media has turned sex into a need, not a want. When, given the pleasurable sensations for both the parties, its primary function is procreation and procreation only. Now coming to your instincts part. Sex is not a primal instinct, survival is... in its pure and raw for. The desire to survive, to avoid death, the will to live...all of these are forms of one main primal instinct, survival. Now, the function you are talking about, has absolutely nothing to do with sex per say.
It is a basic spinal cord function, what you would call a lightening fast response. The brain does not control that at all. For example, when you touch something and its blazing warm, you would automatically, on reflex drop it, and would feel the burning sensation later. This is what we call as quick response reactions. Sex is a very organized act. You know what you are doing, how you are doing it, how you are supposed to proceed. As a man, will you pleasure your partner to a crescendo or not, or are you going to be selfish etc etc etc... The list is just too long.
In fact, if not told, and never actually having the knowledge of sex from any medium, a man wouldn't even know what to do with a woman, if presented to him nude, on a silver platter. The phallus may respond, it will salute, but will the man know? Of course not. This alone disproves your instinct theory. In animals, sex is a PART of primal instinct. It is in their genetic code. Bring two mammals into your house. Keep them away from everything, and watch, they WILL procreate regardless of external influence. Because, they are programmed that way.
Humans on the other hand, have only survival instincts programmed into them, that's it. I am sorry, but you are giving sex too much credit here.