Scientists have now looked at MHC compatibility among romantic couples and they report that the more genes in this system that a couple share, the more sexually unfaithful the woman is and the more she is attracted to other men during in the middle of her menstrual cycle—when she is ovulating and likely to get pregnant.

I guess this explains why women find exotic men irresistible. Read more at; http://helenfisher.typepad.com/helenfisher/2007/02/lovefrom_from_a.html