Me and a friend were arguing about this question: Is sex ultimately for reproduction or pleasure? I said reproduction, but he argues that you can have sex and never have a child, which would prove sex is for pleasure and children are the aftermath of a choice when having sex (to ejaculate and fertilize the egg). Is there any way to clear this up with the logic of evolution (to evolve, one must reproduce)?

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It depends what you mean by "ultimate purpose".

Sex and all that goes with it (the associated pleasures, the urges, the courtship-instincts) has clearly evolved because of its role in reproduction. It wouldn't exist if it didn't play this role. So if something's "ultimate purpose" is to serve that role the playing of which led evolutionarily to its existence, then, yes, sex is ultimately for reproduction.

This is true, I think, even though this notion of purpose is, I take it, problematic on evolutionary grounds. One complication is that a thing (a process, a feature, a characteristic) may be a mere evolutionary by-product, and so not have a purpose in this sense. Moreover, things that evolve for one reason might start to serve new purposes, and persist and spread because of this. And finally a thing's evolutionary purpose(s) (if any) might be entirely indiscernible to us. I presume, though, that none of this applies to sex. In fact, sex is probably the only thing that I (from my armchair) feel safe attributing a specific evolutionary role to. That sex exists because of its evolutionary role in reproduction isn't disproved by cases of unsuccessful copulation, or intentional non-reproductive uses by humans, bonobos and other enlightened beings.

In any case, "ultimate purpose" in this evolutionary sense doesn't have much pull on me when it comes to determining what role an activity should play in my life. Indeed, it's a special ability of ours that we can, and should, find purposes and meanings (be they secular or religious) consciously and reflectively. And in doing this we sometimes, and with some success, diverge from the instinctual urges that move us. Most people, of course, view sex as important for reproduction (though there are now other options). But many also view it as something that can be unobjectionably pursued for non-reproductive purposes--intimacy, novelty, ritual, the sheer physical pleasure... And I see no reason why these can't be justifiably viewed as the various purposes (plural) of sex. (See how easy it is to squander your chances for political office.)

So far, I've been strictly clinical in my answer. I would, though, like to riff off an interesting dimension of your question. It's not uncommon to spend years, even decades, trying not to reproduce while having sex, to only then decide to have children and find oneself in the position of having sex in order to reproduce. This change in intentions can make for interesting differences in the sexual experience itself--some of them quite wonderful and "natural"-seeming. I would tell you more, but this site is rated G.

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