I'm working on my PhD in Immunology, so I found this article fascinating on a few different levels (there's a long version and a short version). Recent research has found, apparently, that women are more attracted to men who are immunologically dissimilar (that is, their MHC* alleles are more distinct) than those who are similar to them. The interesting thing about this is that it is "detected" through scent. The kicker is that this trend reverses when a woman is on birth control pills; she is more likely to be attracted to immunologically similar men than not.
Of course, pheremone and other scent-based research often ought to be taken with a grain of salt, but if it's true, it's interesting. The authors of these articles (and the researchers) speculate that a lot of relationships may experience difficulty because a woman will find herself attracted to her partner while she is on the pill, but after they marry and she goes off of it she suddenly finds herself attracted to different scents. This does ignore the human element in every relationship, but it's certainly feasible that this might be a factor.
I've said for a while now that I think research is going to find, someday, that these birth control schemes are not as healthy for women as we think. It may not be overtly harmful, but there's a lot of subtle things happening in there, and it's going to be those things which get overlooked in short-term studies.
On a different note, perhaps I should figure out how to turn the clothing I wear to bed into a cologne. Apparently the ladies can't resist a big ol' wiff of your MHC.
* - For those of you not in the know, MHC stands for "major histocompatibility complex." The immune system uses it for presentation of foreign components in order to trigger an immune response as well as preventing the immune system from reacting to your own proteins.