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.
As long as the society that the kids together with their gay parents are open minded then I see no problem to that and I ahve watched a real life story regarding a boy growing up has no problem with his foster parent but the real struggle is in his social life which kids would tease him and in much easier word they
bully him. Nonetheless, the story ends that he realized what other people says doesn't matter because what matters is that his foster loves him and he loves him too more than his real parents should.