When does sperm donation equal paternity?

There are lots of issues to be settled about transactions related to reproduction, such as surrogacy, and egg and sperm donation. Some of these are illustrated by a case going through both the courts and the media in California. Here's the NY Times story: Does ‘Sperm Donor’ Mean ‘Dad’?

Here's a relevant paragraph about the existing laws:

"California, like many states, according to Professor Cahn, has conflicting statutes. One provides that any man can establish parentage if he “receives the child into his home and openly holds the child out as his natural child.” But another statute holds that a man who provides his sperm to a doctor for the purpose of inseminating an unmarried friend is “treated as if he were not the natural father” — unless there is a specific written agreement ahead of conception."