No scientist actually knows why or how sexual reproduction came to be, since it is a cumbersome and inefficient means of procreation, or it used to be. The pleasure theory is here emphasized because that is the way things are now, and scientific theories in this area have little existential relevance. It is possible that in the future non-orgasmic pregnancies will become the reproductive method of choice, perhaps for the reason that they may come to yield the best results, genetically speaking. Yet it seems a long shot that sexual activity among human beings will be relinquished, since without such activity there would be no reason for opposite or same-sex genders to bond in a “loving relationship.” And that would be the end of the species.

(http://www.autoadmit.com/thread.php?thread_id=5355497&forum_id=2#49209445)