Yan Liu and Yun’ai Jiang at Beijing’s National Institute of Biological Sciences found that when male mice have low levels of serotonin in their brains they lose their normal preference for female mice and try to mate equally with both sexes.  When injected with more serotonin, their preference for females is restored. 

Read a summary of the work, just published in Nature, here:

 http://blogs.discovermagazine.com/notrocketscience/2011/03/23/low-serotonin-mice-less-choosy-about-sex-of-partners/

and here:  http://sciencenews.org/view/generic/id/71586/title/Brain_chemical_influences_sexual_preference_in_mice