Yes, in the off chance (probably way below 1%) and there's a straight and a gay couple wanting to adopt the same kid at the same time and all other aspects of those couples are equal, I would rather toss a coin than discriminate against one of the couples. Anyway, the scenario you're describing is so marginal I don't see a point in discussing it. You can't have such a marginal scenario dictate the overall policy in this issue.
.
I am willing to bet that if you take a 100 adults, who had been adopted by gay couples as kids, and ask them whether they would have preferred to stay in the orphanage or be adopted and have a family, I am willing to bet that an overwhelming majority of those adults would prefer the adoption option. I have no way to prove that, because since there's no such policy, there are no such adults to ask in this point of time, however I have a strong feeling this would be the response. I can't imagine a kid preferring to stay in an orphanage (be it a Bulgarian or Norwegian one) than being in a home with parents who care and love him/her. No way.
.
One of the most natural things for a kid is to have a healthy home environment and to be raised by people who love and care for him. Whether the kid has a higher chance of turning gay, so what? The kid's future should not be held hostage to the strange belief of this being somehow anti-evolutionary.