Researchers at the University of Lausanne in Switzerland forced a bunch of male fruit flies into monogamous relationships with lady fruit flies, which is against their nature. After a few generations, the scientists noted in the Proceedings of the Royal Society, these flies became dumber and worse at learning. (Basically they taught faithful flies and philandering flies to be scared of a smell; faithful flies were much slower on the uptake and also kept trying to get with unreceptive lady flies.)
It's possible, one scientist not involved in the study told the New York Times, that the constant effort to find mates favors genes associated with learning. But what does this mean for humans who are also animals but not flies, unless you count Jeff Goldblum or Vincent Price or Fly Girls? Does monogamy make humans into idiots? Will generations of monogamous people soon forget to learn? Oh, who knows, man, this is a study about a bunch of flies being shaken around in vials. Chill out, do your thing, you're not a bug.