"The entrenched conviction that children need both a mother and a father inflames culture wars over single motherhood, divorce, gay marriage, and gay parenting. Research to date, however, does not support this claim. Contrary to popular belief, studies have not shown that 'compared to all other family forms, families headed by married, biological parents are best for children' (Popenoe, quoted in Center for Marriage and Family, p. 1)....
In fact, based strictly on the published science, one could argue that two women parent better on average than a woman and a man, or at least than a woman and man with a traditional division of family labor. Lesbian coparents seem to outperform comparable married heterosexual, biological parents on several measures, even while being denied the substantial privileges of marriage.
This seems to be attributable partly to selection effects and partly to women on average exceeding men in parenting investment and skills. Family structure modifies these differences in parenting. Married heterosexual fathers typically score lowest on parental involvement and skills, but as with Dustin Hoffman's character in the 1979 film Kramer v. Kramer, they improve notably when faced with single or primary parenthood. If parenting without women induces fathers to behave more like mothers, the reverse may be partly true as well. Women who parent without men seem to assume some conventional paternal practices and to reap emotional benefits and costs. Single-sex parenting seems to foster more androgynous parenting practices in women and men alike."
-Biblarz, T. J. and Stacey, J. (2010), How Does the Gender of Parents Matter?. Journal of Marriage and Family
Just something to add to the Rebutting Overly-Emotional "Marriage Defense" Soundbites toolkit.
(Tip of the beret: Alas, A Blog)