For the sake of argument, lets say that these studies were actually correct...I'd still say "So?"
What you would need to show me is not that a Man/Man or Woman/Woman couple is worse than a Man/Woman couple....you'd need to show me that they are worse off than a child who grows up the majority or all of their formative years in a foster care.
Because really, the only thing this comes into play in is with adoption. If two men come to an agreement with a woman that one of them will get her pregnant and she'll have their child, it's not the states business. If two women come to an agreement with a man or a sperm bank and get themselves pregnant, again, it's not the states business.
At least with adoption, there's some kind of transaction of sorts that reasonably could be regulated. So that's where the question comes down to. And so long as there remains children that need to be adopted, and aren't, then I would strongly disagree with disallowing any loving couple from being able to adopt if they meet all the standard requirements. And that would be my stance until such time that you can significantly show that their adopting would actually be more harmful to the child than keeping them in foster care.
There are dozens, if not hundreds, of potential factors that may go into making a "less than ideal" set of parents that we in no way, shape, or form test for with adoptions and in which the state absolutely doesn't step in and use as a means of keeping the couple from becoming pregnant. There's no reason why homosexuality should be the weird exception.
So for me, all these studies, on either side, are basically just dick waving contests in hopes of convincing people who likely will never be convinced on either side.