Great question. I'd figure it would be somebody who is EXTREMELY uncomfortable/afraid of their own sexuality, somebody who feels beaten down by the sex they are attracted to, someone who has a disdain for the sex they are attracted to, and quite possibly someone who was abused horribly physically and/or emotionally.
Kind of a nature vs nurture thing with this and other socially unacceptable activities like murder or theft. Some criminals are born some are created...kind of scary.