Gay marriage and gender socialization: (From the comments of this post): As gay marriage becomes widely accepted, people will feel the need to stop believing that a child benefits from by being raised by a mom or a dad. For there to be no disadvantage with a child being raised by two moms, being raised by a dad cannot be considered important. For there to be no disadvantage with a child being raised by two dads, being raised by a mother cannot be considered important.
Even gay families worry about their child lacking a parent of one sex (e.g., lesbians wanting to find a father figure for the boy). In terms of gender socialization, being raised in a homosexual home is the same as being raised in a single-parent home. There's all sort of research documenting the problems that stem from growing up without a mother or a father; problems that are not simply the result of not having two adults present. Kids still fare poorly if they are raised by mom and grandma--two adults.
And mom and grandma at least have the advantage of a biological relationship to the child. Gay families have at least one non-biological parent. Research shows that non-biological parents are not as good as related parents.
The fact that stepfamilies are just about as bad for kids as single-parent families shows that "two grown-ups in the house" is not enough. A child seems to need his biological dad and his biological mom.