Jill and Jane have a child, but Bob wants visitation and parental rights. Do all three names go on the birth certificate?
A few years later Jill and Jane separate, Jill gets possession of the child, do Jane and Bob both have to pay child support?
Your example is moot, and completely backwards.
Adam and Eve want to have a child. Adam happens to be infertile. Steve is an anonymous donor. Steve and Eve are the biological parents to the child (even though Adam and Eve don't know him). Steve signed away all rights to the child as a donor. He may get updates if Adam and Eve decide to do so.
Regardless of what happens to Adam and Eve's marriage, Steve can't suddenly jump in and claim legal rights to the child, nor can Eve suddenly claim that he needs to pay child support.
Take the above example, and replace the infertile Adam with a woman named Jane. Same scenario, homosexual parents. Same legalities.
Take any existing heterosexual scenario with an infertile couple, or a couple that adopts for any other reason (I have some friends who adopted after their biological child died). Now replace one of the parents with one of the opposite gender, forming a homosexual relationship. What makes that scenario so much different than the infertile heterosexual scenario, aside from your prejudice against the "lifestyle"?