I'd like to see those studies :3
You know, a quick Google doesn't turn up any evidence to support that, and I can't remember where I read it, so I'm going to back down from that claim.
However, a quick Google DOES find numerous studies that suggest same-sex parents are no different from heterosexual parents, and
WebMD cites studies suggesting that two gay parents are better than one straight parent.
It's less that 'they really really want to be parents' and more 'they're scrutinized much, much, much more'. When your primary means of getting a child is through adoption, you're a lot less likely to get a kid as a complete waste of humanity. My brother makes 40k/yr through the military and has a wife who was willing to be a full time mom and they weren't able to adopt. If a gay couple isn't beating that by a lot, they're never going to get a kid in the first place.
That's a fair point too. Maybe I should be looking at studies of adopted versus natural parents.
Anecdotally, my grandparents have neighbors who are a lesbian couple; each one has a natural daughter (I assume via artificial insemination but I haven't asked) and they're currently working to finalize the adoption of three more kids. It's been a very difficult process for them, but they've really worked hard and have done a great job and by all accounts the adoption agency has scored them very high and really gone to bat to help them get approval.
So again, that's anecdotal and I can't exactly say they're representative, but I'm willing to bet that, at least in as gay-unfriendly a state as Arizona, their story's a common one and you really have to kick a lot of ass to get to adopt. (They also say it's gotten a lot easier since Napolitano took office.)