You are trying to avoid the difficult choices.
I have not brought up the topic of donation, that has been other posters trying to get around the tough choices.
This thread is about parental rights. If someone is a donor, then they have no rights.
Sigh.
Again, assuming that the biological father is known and does not willfully sign over his rights then he has not lost any paternal rights. He will, until the child is 18, have the right to petition the court and get all of his legal parental rights as the court sees fit in the best interests of the child.
In the above case no other person has ever gained legal parental rights to the child, period. Full stop. End of discussion.
The only way for two women or two men to both be legal parents of a child is either some type of sperm donor or through legal adoption in which the court removes, either by force or due to the biological parents willingness, the biological parents rights and gives them to the adopted parents. That is what the entire purpose of adoption is. There might be another scenario but I would imagine it would be very rare and vague.
This is not the issue that you are making it out to be. Lots of hetero couples go through the exact same process every year.