The California Supreme Court has ruled in 3 same-sex custody cases involving parental rights/responsibilities:
In Elisa B. v Emily B. the Court held that there could be 2 mothers under the principle that a parent who accepts the non-biological child into his/her home can be liable for child support. Note that this was a state child support action which often results in an outcome that protects the public treasury.
In Kristine H. v. Lisa R. the court refused to let a biological mom who stipulated to a pre-birth parentage judgment to collaterally attack it.
In K.M. v E.G. the Court ruled that where a woman donates her egg to her partner and they raise the child, that the child has two mothers, regardless of the fact that the donor mother signed a consent/waiver of rights at the hospital, enforced under the California insemination statute. The waiver was found to be irrelevant.
In each case, the end result is that the children have two legal mothers. You can find the cases at the California Supreme Court’s website. If you’re really bored, you can also watch the arguments using the same Court web site.