Did you ever read this article?
http://www.foxnews.com/health/2011/11/09/former-rugby-player-claims-stroke-turned-him-gay/
You can’t extrapolate from a single instance, of course, but this makes it sounds like a defect of some sort.
It’s stunning how far the left has coming in defining down deviancy. If they succeed in normalizing homosexuality, I have little doubt that they’ll move on to polygamy, pedophilia (as long as it’s “consensual,” they’ll say), and bestiality.
Most likely it's both and a few other factors. In vary degrees for different individuals a permissive culture, nurturing (i.e. an abusive childhood), a more feminine (but not necessarily homosexual) physiology, lack of attractiveness as a mate (think bull dyke) and, of course, an element of will are likely factors.
"The fallacy of the single cause, also known as causal oversimplification, is a fallacy of questionable cause that occurs when it is assumed that there is a single, simple cause of an outcome when in reality it may have been caused by a number of only jointly sufficient causes."
http://en.wikipedia.org/wiki/Fallacy_of_the_single_cause