An article from New Scientist talks about a new study finding that women prefer odours from potential partners who are genetically dis-similar. This contrasts to earlier studies that claimed women prefered the faces of men who were genetically similar.
Apparently, it all has to do with "the major histocompatibility complex (MHC) - [har har] the huge molecule on cells, unique to each individual, which helps our immune systems to distinguish native from alien cells....The underlying theory is that humans avoid the dangers of inbreeding, and maximise the chances of having genetically fitter children, by selecting partners who have a vastly different MHC from their own."
I think it's rather silly to say "women prefer this, men prefer that." Human diversity is so vast. I think certain people are inclined to find people who are genetically dissimilar and somepeople like to keep it in the family. Humans specialize like bees. There are so many different strategies.