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.