Wasn’t there a big court case a few years ago about a guy who found out he wasn’t the father and didn’t want to make child support payments? I believe he still had to pay anyways. IIRC it was a big story here at FR.
That's not unusual at all. Basically, if a man accepts paternity, he is the father in the eyes of the law, even if it is later discovered the child was sired by someone else. Doesn't make it right but it is what it is. This legal interpretation actually is founded in English common law.
Don’t worry! Not long from now the courts will give women with children by sperm donor the right to sue men who donated sperm at a clinic.