Women on the Pill pick bad mates.

According to this LiveScience article, women typically choose men who smell different from them, because there's more genetic compatibility that way, or something or other. But when women start popping the Pill, they become attracted to guys who smell more similar to them.

From LiveScience:

The study involved about 100 women, aged 18 to 35, who chose which of six male body-odor samples they preferred. They were tested at the start of the study when none of the participants were taking contraceptive pills and three months later after 40 of the women had started taking the pill more than two months prior.

For the non-pill users, results didn't show a significant preference for similar or dissimilar MHC odors. When women started taking birth control, their odor preferences changed. These women were much more likely than non-pill users to prefer MHC-similar odors.

"The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odors," Roberts said.

I like this article because it provides scientific explanation for why loose women hang out with jerks, and it also infers that jerks smell like girls.