Once again, don’t take the argument at face value and try to win on the grounds they’ve already established - they’ve intentionally skewed the assumptions and the measures of “wellbeing” to favor their preconceived conclusions. What you need to do is to delve into how they defined all of the terms of art they bandy about - like “wellbeing” - and what sort of assumptions they rely on without really stating as such.
For example, I don’t think that most of us would consider over-indulged little brats to be well-adjusted or properly raised, but (and I don’t know if this is one of the subtle biases in this particular study) if one defines “well-being” in such a way that it increases to the extent that a child gets what they want - then having two over-indulgent mommies may result in a supposed increase in “well-being” when all it really indicates is that the child is especially over-indulged and especially likely to be a self-centered, childish brat when grown up (that is, a standard-issue liberal).
I think I found another blogger who agrees with you:
“I haven’t read the study, which comes out in July in the journal Pediatrics, but I’d venture that childhood outcomes have little to do with sexual orientation per se. For gay parents, having a child is a more “involved,” and in most cases expensive, process. It generally involves hefty adoption fees or paying for assisted reproduction technology. So those who have children are a self-selecting group: They want children and are wealthy enough to either meet adoption requirements – and pay the fees – or pay large medical bills. It’s no surprise that the lesbian parents in the study were very involved.”