Well, now look at it... gay people adopting babies thrown away by (presumably) straight people. Anyway, so there were actually studies and those found out that there were no negative effects? Good then. Orphans need families and allowing gay people increases the number of possible parents for them.
I think this became possible in Western countries because they got started with gay rights way earlier than others. Not sure if this tolerance will continue though, with right-wing leaders becoming more popular. This would take a long time to reach countries that currently don't allow same-sex unions. Probably at least 50 or so years after gay marriage is allowed would be the proper time to make adoption available, IMHO. That should allow time for some of the more bigoted in the population to die off first.
I really wouldn't suggest immediately allowing adoption right after same-sex marriage approval. The assholes might focus their anger on the kids instead (since they already failed to stop the parent marrying).
The main argue I've heard is that the child will fare badly as a result of it. Whether it is true or not is another question, but I think I saw a local study some time ago that gave that argument some credit. I'm on phone at the moment, so I don't really feel like trying to that study now though (and it wouldn't be in English anyway).
Personally, I am a little bit hesitant, but if it can be proven that a child growing up with parents of the same sex will have an equally good childhood as those coming from a "normal" family, then I do not see any problem with it.
The kids will only fare badly if the society their family belongs to look down on homosexual couples.