Kieran, that's a HUGE can of worms that no one in this thread can answer. It's not as simple as "does environment affect behaviors?" Of course it does. What you're specifically asking is "Will two gay parents be more likely to have a child that is gay?" The comparison to an axe murder is completely invalid, even in the sense of the environment comparison...assuming the environment that helped breed the anti-social behaviors was an abusive one.
Of course, I don't really have a problem with anyone being gay - nor do I have a problem with a gay couple raising a kid...even if they might influence the kid's sexuality. Though I don't think that'd be possible unless the kid had leanings toward homosexuality in the first place. All behaviors can generally be linked to genetics and environment, and I don't doubt this is any different. But the natural urge to procreate is pretty strong and unless you've already got the instinct build in that you dig the same sex, I don't think anything short of a terribly abusive environment is going to deviate you from what's natural for you. Frankly, I think the only reason a gay couple might be more likely to have an openly gay child is the fact that the child won't have a fear of being shunned by his loved ones. Of course, that's just my opinion - as I said, I don't think anybody here knows for sure.
And on the subject of axe murders, I don't know what "camp" i'm in, but if you kill someone without a damned good reason (ie: self defense or the like), you've given up your right to live with the rest of us. I really don't care if you had a bad childhood, are crazy, retarded, or whatever, you simply aren't fit to mingle amongst the rest of us.