Perhaps the answer lies in this study:
http://www.sfgate.com/cgi-bin/articl...w080428D07.DTL
The study finds that men with multiple older brothers are more likely to be gay:
"Bogaert said the increase can be detected with one older brother and becomes stronger with three or four or more."
When I first read this I thought, "Sure... because younger brothers have a higher chance of being molested by an older brother, and getting molested has a funny way of messing up your sexual wiring." But that isn't the case, as the report shows. The evidence suggests a prenatal link.
Since Mormons have such big families, this might suggest why there are so many gay Mormon men.