My original point was that copulation is for the purpose of reproduction. Physiologically, sex is for the purpose of reproduction. Any other meaning beyond this is philosophical, not biological. Since Homo sapiens is a species which mates essentially for life, and since sexual pleasure is a bonding mechanism that strengthens the relationship of the mated couple, it seems rather obvious that the ability to experience pleasure from sex has been gained through natural selection. The more it is done, the more offspring are produced.
Sex certainly feels just as good to unmarried partners as it does to married couples.
Also, your argument isn't reflective of the reproductive patterns of practically every animal species on the planet. Females of most species go into estrus and become receptive normally only from once to a few times each year. Copulation with their mates must occur during these rare periods or there will be no offspring produced that year. There is no pleasure aspect at all that drives this instinct, it is purely a matter of species survival. Even if those species engaged in constant copulation throughout the entire year, there would be no increase in the number of offspring.