Opposing gay marriage to protect the children?

One frequent heard argument to object to gay marriage is that it is better for the children. lets assume for the moment that it is indeed true, despite that it is clearly not. Is a ban on gay marriage actually having an effect on where those children grow up? Not at all? The only difference is that they have two parents who are not married and have no rights, but for the rest, they will have two fathers or two mothers regardless. So, why oppose gay marriage if prohibiting gay marriage does not have an effect on where the children grow up?