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.
Yes, that is entirely true and I think that is the main issue. It is, however, a reality in many places of the world. And while it is indeed bad to punish gay people for other people's wrongdoings, the question remains whether or not we should expose the children to the prejudice and perhaps in some cases bullying that could come with having with having same sex parents.