I'm not entirely convinced by the genetic factor simply because I know twins, and one of them is gay and one isn't. Now, I've never met the straight one, so he could be closet or something. I've been told that he's a real bro douchebag, so he could be doing the "rebelling" thing.
I believe that part of it comes from genes and part of it from nurture/environment. I believe the nature part is simply natural genetic predisposition (like hormone levels [which are different for nearly everyone]) and part from experiential predisposition.