It doesn't matter. Marriage is marriage. Answer the question.
Let me correct something for you. Based on your example, if the couple has to go to a sperm bank, one would assume it means the male is where the problem lies. The only biological parent in that situation is the female. That being the case, for the male, it isn't HIS child. To say "their" child is factually incorrect from a scientific standpoint.
If the couple is homosexual, the only way that can happen is with lesbians. One lesbian can get pregnant from a sperm bank but the other one can't provide the sperm. Therefore, it isn't the non-birthing lesbian's child. It's the one that gave birth and sperm donor's child.
Marriage isn't marriage. No matter how much you freaks want to believe so, your unnatural marriages to the same sex will never be to the level of my normal heterosexual marriage. You don't have to like it as that will never be a requirement for it to be so.