i think Forrest Gump was right.
LT Dan said it was his destiny, while his mom said it was random and free will.
He said it was probably a bit of both.
By this it could be partial genetic disposition and partial upbringing for some cases, where as others could be completely one or the other. Different people respond to the same thing differently. And until science can completely map a persons mind electrically and chemically at any given moment(which would make it possible to 100% predicting their actions to anything in the world around them) id say the best answer would be probably genetics with upbringing. In reference as well to the animal kingdom, there has been plenty of documentation of gay animals. And not in as confused puppy humped the wrong one. Dolphins, which socialize, go on dates, hold flippers, and do each other in the blow hole have been observed to have gay and straight, as well as bi members of their species