In a recent parsing of census data to look at kids held back a year in school, the answer is yes, if you control for parental education. Having straight parents didn't help compared to gay parents, but what did help was
having parents, foster or real, not a group home,
there being two parents, and
the parents being married to each other.
The number that struck me in this article was that Those who were awaiting adoption or placement in a foster home were held back about 34 percent of the time, compared with 7% of kids of straight married couples. That's 5 times higher. Some of it maybe cortisol secreted during the stress that brought them to foster care, but for whatever reasons, we're really failing our foster kids.
And it's another argument for gay marriage. As though I needed another one.