In the case of your friend, I would venture to guess that his experience led to sexual conditioning. I think this does happen and it depends on many factors as to the overall strength of influence. Some are: the innate personality of the victim, what age this occurred, if there was any brain washing techniques used (most of the time there is with molestation cases), and if there were any positive influences to counter this negative one.
Yes, I do believe that someone can be negatively conditioned to be gay. However, I do not believe this is true within a loving, caring environment, such as a gay-parent household.
Again, there are many layers to human sexual development. I find it all pretty fascinating!