A nifty experiment put men in a room with women who were at various stages of their monthly fertility cycle - unbeknownst to the men. The men were then asked to rate the women's relative attractiveness.
One interesting finding is that men rated women differently depending on where the women were in their cycle, even though there was no obvious visual sign of their fertility.
The more surprising finding is that the single men found the most fertile women most attractive, but men in a relationship found the most fertile women least attractive.
This seems to me a subtle chemical defense of marriage that is going on in men.