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I wonder why (especially human) sex lasts longer than just a few seconds?

My thought is that the shorter it takes a man to ejaculate, the lesser is the risk that he gets interrupted by a rival or a predator. Therefore, men who ejaculate quickly get an evolutionary advantage in the long-term.

But, as human sex lasts several minutes, I'm obvioulsy wrong with my argument. Anybody knows why?

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    $\begingroup$ there is probably an element of recreation into your procreation argument, which can contribute towards this. $\endgroup$ – Behzad Rowshanravan Nov 1 '14 at 18:30
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    $\begingroup$ If male sexual performance / stimulation / duration affects females in terms of things related to fertilisation (immune systems, gene expression etc.) Then there is possibility of selection for duration $\endgroup$ – rg255 Nov 1 '14 at 18:38
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    $\begingroup$ It's also entirely possible that there are physiological constraints on the reproductive system as it happened to evolve that necessitate a certain minimal duration of sex. $\endgroup$ – seaotternerd Nov 2 '14 at 4:38
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    $\begingroup$ @anongoodnurse "A few seconds" is below the 2.5 percentile as estimated here. The median they obtained was 5.4 min. $\endgroup$ – alephreish Nov 2 '14 at 20:46
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    $\begingroup$ @anongoodnurse Why should we multiply? The question addressed was about boundaries of the norm and how to define pathology. What's the base for your pessimistic claim? By the way, those observations are also related to seaotternerd's idea: "a few tens of seconds" is physiologically possible, but for modern humans it is certainly not normal. $\endgroup$ – alephreish Nov 2 '14 at 23:38
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There are plenty of animals with brief procreative periods, as you suggest. In chimps, sex takes 10 to 15 seconds - much less time than humans. So the question is really why have humans evolved to be different?

I don't think any of the answers based on fitness are particularly compelling. It's not like sex feels like running a marathon or less physically fit people can't manage to ejaculate in their partners so it seems unlikely that the lengthy sex act serves to distinguish fitter individuals.

Instead, I think the difference probably lies in the pair-bonding role of sexual activity. Humans form unusually long-lasting, typically monogamous or largely monogamous, bonds that function to provide the support needed during the lengthy childhood that humans undergo. The prolonged, and highly pleasurable, sex act likely functions to help maintain these bonds in order to keep the couple together and provide the stable support needed to maximise the chances of successfully raising a child to adulthood.

Unfortunately, to the best of my knowledge, there is no high quality empirical evidence that would convincingly answer the question so any answer must be somewhat speculative.

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I'm going to give a tentative answer full of speculation and guesswork, but it's too long to fit in a comment so here goes.

Sex duration is possibly a sort of human reproductive handicap. Sexual arousal can soften the cervix, increasing fertility during lengthy sex sessions. Lengthy sex exposes them couple to predators and takes time and energy, so if you have the calories to burn you are probably a better mate.

The more orgasms a male has had in the recent past the longer the average sex session takes to ejaculation. This kind of makes sense as an adaption strategy to different frequencies of sexual intercourse (if less sex happens it's more important to ejaculate quickly, if more sex happens you want to get as much fertility out of the sperm you produce as possible).

Comparing testicle size and penis size to other primates, humans have a large penis and relatively small testicles(compared to chimps, say) which implies we spend more energy on the sex and less energy on the ejaculation than chimps. It also implies a lower sexual frequency or smaller ejaculate sizes or larger vaginal depths or really a number of other things.

The whole system kind of makes sense. Long sex durations are a sexual handicap system to show the fitness of partners, which are rewarded with slightly higher fertility. The whole post-sex evolutionary mechanism thing is a layer on top of the underlying mate selection which is beyond the scope of a book, nevermind a stack exchange answer.

That's probably at least a little bit wrong. The handicap principle isn't super well accepted as a general evolutionary principle, even though it explains some things.

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    $\begingroup$ -1. For the handicap principle to be valid, potential partners should have a way to distinguish the duration of sex length prior to mating. This doesn't appear to be possible. If partners are unable to distinguish the length prior to mating, no sexual selection takes place. $\endgroup$ – March Ho Jul 24 '15 at 3:33
  • $\begingroup$ I think you misunderstand. The selection is performed by the cervix, not by the partners. Longer sex durations (weakly) imply a higher sexual frequency and that in turn (weakly) implies higher fitness. A partner that has a great deal of sex is a good partner, because your offspring are likely to have a great deal of sex. If a sex act lasts a long time, the cervix is interested in increasing the per-sperm fertility for this surprisingly lengthy sex act. $\endgroup$ – Resonating Jul 28 '15 at 13:37

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