Birth Control Pills Affect Women's Taste in Men: Scientific American

It's all about scent. Hidden in a man's smell are clues about his major histocompatibility complex (MHC) genes, which play an important role in immune system surveillance. Studies suggest that females prefer the scent of males whose MHC genes differ from their own, a preference that has probably evolved because it helps offspring survive: couples with different MHC genes are less likely to be related to each other than couples with similar genes are, and their children are born with more varied MHC profiles and thus more robust immune systems.
A study published in August in the Proceedings of the Royal Society B, however, suggests that women on the pill undergo a shift in preference toward men who share similar MHC genes.