From Nature:
Women beware: instinctive preferences might up the odds of getting pregnant when cheating on a partner. In a study looking at the ever-interesting (and ever-mysterious) question of why women are attracted to certain men, researchers found that sexual interest shifts with a partnered woman’s menstrual cycle. When fertile, women in relationships are most attracted to single men; when infertile their attraction shifts to coupled men.
The reason, the researchers suggest, is that coupled women who are thinking of having an affair (even when asked to think about it by researchers) subconsciously select a man who is more likely to be a willing partner when they are fertile. Courting a coupled man may be both a waste of time — as he is less likely to participate in an affair — and hazardous, as there is a greater chance of getting caught. “Ancestral women who felt more attracted to a single man than to an already coupled one would have been more likely than others to succeed and transmit this preference to their daughters,” says Paola Bressan of the University of Padua in Italy. “These subconscious preferences are apparently still with us.”
More here.