A study, which is supported by research in the 90s, finds that women on the birth control pill will choose mates that have a similar smell make-up to themselves, which evolutionarily is a bad thing; in a normal hormonal stage, women choose guys with contrasting smells (major histocompatibility complex (MHC) gene-produced odor to be more precise) because essentially it means that the woman is not procreating with a distant relative and her offspring is more genetically fit, and will also recognize more smells as familiar and be more open to people. Once women go off the pill, apparently they are more likely to leave or cheat on the similarly-smelling partner.
At first this totally blew my mind! I knew that smell played a MAJOR role in mate selection, but the fact that the pill can mess with one's sense of smell and ability to smell others to the point of evolutionary malfunction is amazing. Although upon further analysis it makes perfect sense, since the pill is a hormone replacement, and there is already plenty of evidence that a woman's hormonal cycle affects her sense of smell. Women have a hard time becoming sommeliers (wine tasters) because their sense of smell (and therefore taste) changes throughout her menstrual cycle so they are considered not as reliable tasters as men. Many women I know couldn't walk down the soap or seafood aisle at a grocery store when they were pregnant because their sense of smell was just too sensitive.
It makes me wonder what this means for women who have been on the pill from the time they were teenagers until they decide to have their first kid. Obviously this doesn't mean that every woman who meets her mate on the pill will dump him as soon as she's off (I've been with my guy for five terrific years), but the implications of this are fascinating.