Article: "The truth about relationships and the pill"

http://abclocal.go.com/kgo/story?section=view_from_the_bay/sex_relationships&id=6420862

Some quotes:

"Subconsciously, it turns out most women gravitate toward the smell of men who have the polar opposite immune system, and thereby opposite smell of their father's and brother's. But here's the Pill connection: Women who were on the pill were most attracted to men who smelled like their father or brother."

"To take it one step further, studies from fertility clinics report that embryos conceived by parents with dissimilar MHCs are more likely to make it to term than are those whose parents have significantly similar MHC genes."

"Conclusions reported that women with dissimilar MHC genes to their partner tended to be more satisfied, responsive and adventurous in the sexual arena. In fact, during ovulation they experienced more orgasms. Women with MHC similar mates, reported more fantasies about other men, especially during their ovulation. They also reported more infidelities."
Christine Garver-Apgar, a psychologist at the University of New Mexico, links relational difficulties in the long-run with heterosexual couples to similar MHC profiles. Garver-Apgar reports the number of MHC genes couples shared corresponded directly with the likelihood that they would cheat on one another; if a man and woman had 50 percent of their MHC alleles in common, the woman had a 50 percent chance of sleeping with another man behind her partner's back."

"Many women are on the pill because they want to enjoy a certain amount of sexual freedom. Ironically, these women may be pushing away potential suitors based on their suitor's olfactory preferences. The pill prevents the ovaries from releasing an egg, fooling the body into thinking it's pregnant. So women on the pill, may be eliciting odors of being pregnant and since pregnancy is such a vulnerable state, it seems to activate a preference for attracting those who are genetically similar to us and are likely to serve as protectors."