It would only make sense that sex and pleasure work together. I mean if it hurt like hell at all times, you would not participate. And if you did not participate, and your perception of the act via your nervous system said damn that hurts and this particular perception was genetic... Well ya just would not make offspring. And if you went thru the painful act because it was pleasurable for another individual... well your offspring might inherit the "pain gene"... and they might be less likely to engage... and on.

So it makes perfect sense that some sort of reward (other than offspring which are not instantaneous products) would come along with the act from an evolutionary point of view.

This is what I tell my wife anyway.

Come to think of it, maybe I should shut up. Strike the above.
Leave the logic part out of it and just go at it.