Here’s something interesting: Studies suggest that females prefer the scent of males whose histocompatibility complex (MHC) genes differ from their own, and birth control pills can affect this preference. A woman on the pill might be more attracted to a MHC-similar man; but when she goes off the pill, the accompanying hormonal changes will draw her stronger toward MHC-dissimilar men.

Reference: http://www.scientificamerican.com/article/birth-control-pills-affect-womens-taste/