Ample evidence that boys and girls do better with a father and a mother, compared to just a mother. Are there studies that show that two mothers are worse than a mother and a father (or just a father for that matter, if we believe that the male role model is significant factor).
I don't have the studies at hand but did come across research articles earlier that suggest that girls are only mildly affected in a female-only parenting situation whilst boys are strongly affected.
It makes sense as they have no male role models. Not at home and close to none in most of their schooling trajectory.