Evolutionary psychology is a can of worms to begin with. If you want to find a particularly violent online argument, search out two people -- one of whom believes mental and behavioral differences between men and women are entirely a social construct, and the other of whom thinks it's possible that men exhibit more big-picture thinking (yet have no idea what color the bedspread is) because those who could accurately scan the horizon for game survived to pass down that gene along with women who observe small details like whether a mushroom is poisonous or whether a baby is sick and develop communication skills to share that information.
That's just an example. But last month, the folks at The Journal of Evolutionary Psychology determined that, although the female orgasm doesn't get a woman to lie there and happily retain sperm at the optimum moment (just one of the fascinating speculations we recall from Desmond Morris' 1967 The Naked Ape, along with a theory that large human breasts and pigmented lip tissue mimic the back side of a lady, thereby encouraging face-to-face sex and less promiscuity), men whose relationships are threatened by "sperm competition" (a.k.a. Sancho) perform more cunnilingus. Oh, and they do it right. Well. Successfully. Whatever.
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The authors (Todd K. Shackelford, Michael N. Pham, Yael Sela, and Lisa L. M. Welling) did acknowledge the limitations of the research (although, oddly, not their criteria for whether a man is at greater risk of competition-induced cuckoldry, which include an attractive partner and/or absence from her) and that alternative hypotheses might explain why heterosexual couples engage in this pointless behavior.
They were really shooting for (so to speak) evidence to support the sperm-retention-during-orgasm thing, so it's not as though the scientists really had no idea why not-directly-procreative sexytime is fun; they were just wondering about something else and are probably a little bummed that they were wrong.
Also probably spending too much time at the lab, they or their significant others may be telling themselves right about now. Though the two male and two female researchers all work together at the same Michigan university, so who knows, really?