This news that shouldn't surprise you if you've been paying attention: Children raised by lesbian couples tend to do better on average than children raised by straight couples. There are probably a million reasons for this, but the first that comes to mind for me is that it might just be better on average to have two moms instead of just one. That's two people who've been socialized from birth to identify as nurturers.
Gay or not, your average woman has had a lifetime of experience in the neccessary-for-parenting arts of boosting self-esteem, monitoring loved ones to see if they want for anything, and even minor things like choosing food for nutritional value instead of taste. Obviously, individuals will vary, but few women, regardless of sexual orientation, escape the gendered training to put others before yourself. One of the things that a sexist society does wrong by men is discourage them from learning these skills, and sometimes even shames them for doing things like caring too much or having feelings. For a lot of new fathers, there can be a steep learning curve in learning these basic skills. Some do a bang-up job of reaching the minimum mommy level, but some don't even bother, creating generation after generation of TV writers churning out story lines of grown adults feeling estranged from fathers, among other things.
The ugly truth is that women do more housework -including child care-than men, even in our supposedly enlightened time. When the average straight couples has children, her housework increases three times as much as his. Again, individual mileage may vary, but on average, it seems replacing men with women simply means more work (and nurturing) gets done. And children benefit.
Conservatives who trot out the "children need a mother and a father" line have a very specific and erroneous belief in play-that children are better off the closer their family hews to a traditional model in which the mother is submissive to the father, the mother does all the nurturing and the father does all the discipline, and children learn that gender roles are rigid opposites. I suspect the opposite is true, and that children do better if gender roles are flexible and all adults in their lives are nurturers. A similar study that compared traditional nuclear families with ones in which the fathers try to take on more of the mothering role would probably produce similar effects as this one that compared lesbian families to straight ones.