In a recent case in Pennsylvania, a male friend of a lesbian couple who acted as a sperm donor so that they could have children together was ordered to pay child support upon the dissolution of the lesbian couple's civil union because he took a relatively active role in the children's lives.
This seems like a clear-cut case to me: The children had two legal guardians. These legal guardians had a legal arrangement (civil union) binding them together. Then, the legal guardians dissolved the legal arrangement binding them together. Upon this dissolution, one parent got custody of the children, and the other parent was required to pay child support.
Nowhere in this is there room for another parent to be added to the equation just because the civil union is dissolved. Yes, the man was the biological father of the children, but he should never have had legal rights to the custody of the children nor responsibility for their care, because he never took on the role of legal guardian.
The argument from the court for why the man was required to pay child support was because "he was so involved with them. ... They called him Papa," and he spent "thousands of dollars on the children, including purchases of toys and clothing."
I don't personally see how this reason validates requiring him to pay child support. Many grandparents have spent thousands of dollars on their grandchildren and purchase toys and clothing for them. Some children call their grandparents variations on "Papa"- I personally know a girl who calls her parents "Mom" and "Dad" and her grandparents "Mama" and "Papa." If a hypothetical couple obtained a divorce and Parent A got custody of the children, should Parent B's parents pay child support? Certainly not, even if they were addressed as "Mama" and "Papa" and had spent thousands of dollars on the children already. This seems intuitive.
Could this be indicative of prevalent but understated gender assumptions? "Surely, two women cannot support this child on their own; we must find a man to provide the traditional caretaker role and provide the financial support necessary!" Sure sounds possible ...