I laughed when I read the first sentence in a story in the Metro last week:

When having sex with a woman, most men would be turned off if they knew another man was nearby.

Mind you, it may just be the people I know. ('If your friends were normal people, they would not know you', remember, though it looks like the Nature article has vanished.)

Oh, yes – the story was that male meadow voles react to the presence of other male meadow voles by producing more sperm…