Whatever happens, that child won't have a "normal" childhood. It's almost an inevitability that he/she will be bullied or teased in some way at school, and I really wouldn't suggest homeschooling as an equal alternative. By taking a child out of the education system and not giving them the chance to mix with people of their own age, you've effectively ruined any chances of them having a normal childhood already. While I'm sure some people would come out of that background as perfectly normal and well-integrated adults, I think that a lot will/would feel far more socially isolated than if they were to have a standard family background.
And at the end of the day, the reason gay couples would want to raise children will be almost completely selfish. Unless the child they adopted had a troubled background, I can't see any other reason as to why they'd want to adopt than to satisfy their desire to have a child. Is it really right to put said child through it all, just to satisfy those needs? In 20-30 years, when it's more socially acceptable to be gay, maybe. But right now it'd be a mistake, imo.
Edit: Added to this, is whether or not the child would have the ability to relate to people of the opposite gender. If a boy grows up with 2 gay men as parents, I'm sure they'd have trouble relating to girls.