"These studies ... are impressively consistent in their failure to identify deficits in parenting abilities or in the development of children raised in a lesbian or gay household."These are negative results, and they do not mean much. They applied simple-minded methods, and failed to get results.
Here is the abstract to one of those studies:
This study examined associations among family type (same-sex vs. opposite-sex parents); family and rela-tionship variables; and the psychosocial adjustment, school outcomes, and romantic attractions and behaviors of adolescents. Participants included 44 12- to 18-year-old adolescents parented by same-sex couples and 44 same-aged adolescents parented by opposite-sex couples, matched on demographic characteristics and drawn from a national sample. Normative analyses indicated that, on measures of psychosocial adjustment and school outcomes, adolescents were functioning well, and their adjustment was not generally associated with family type. Assessments of romantic relationships and sexual behavior were not associated with family type. Re-gardless of family type, adolescents whose parents described closer relationships with them reported better school adjustment.They sampled a few families, gave them some questionaires, and found that they all had about the same self-esteem and other subjective self-reported indicators.
To see how weak this is, imagine if someone studies NFL football teams by sending questionaires to players and coaches, and asking about their self-esteem. He might conclude that all of the teams are about the same, and miss the fact that some are winners and some are losers. He would probably miss a lot of other differences also, especially if he is not a football fan.
The problem is deeper. Psychologists and other social scientists publish studies all the time, but there are hardly any studies showing that any parental strategy is better than any other.
Everyone who has kids, and most of those who don't, have opinions about how best to rear kids. And yet those studies are not backed up by research. Most of the opinions given by court-appointed child custody evaluators are also not backed up by research.
The only studies that say anything useful about parents are those that say that kids do better with their natural dad and mom. There are dozens of such studies, and they give specific quantitative measures showing how the kids are observed to do better. No one disputes these studies.
Some readers may comment, "I was adopted, and I turned out great", or "X was a child abuser, and her kid was better off in foster care". I do not doubt that there are many such examples. The studies apply to statistical samples and majorities, and not to every single case.
I am just reporting on the studies. If you don't like the studies, complain to the authors. If I have overlooked some studies, then go ahead and point them out in the comments.