Interesting!

Something else fascinating about sperm competition, by the way, is that sexuality experts have studied the human male response to the close presence of other sexually aroused men and learned something very interesting.

When a man sees another man’s erect penis in the context of having sex with a woman (or thinking about having sex with a woman), his own penis is likely to become more erect, and he is likely to ejaculate harder and farther.

Interestingly, this may partially explain why so much porn for men features men and women instead of just women.

But it seems also to give us more biological evidence that women did not evolve as monogamous.