COURT: Twin Girls Have Two Fathers

But this isn't a case involving gay men. Via CNN:

A New Jersey father has been ordered to pay child support for one girl in a set of twins after DNA tests proved he is not the father of both, according to court documents. The mother testified in a paternity case that, within a week's time, she had sexual intercourse with two men -- the man genetic tests confirmed as the father and another unidentified man. The judge acknowledged the unusual circumstances of the case in a ruling this week. "This is a case of first impression in New Jersey and only a handful of reported cases exist nationwide," Superior Court Judge Sohail Mohammed said in his ruling. The ruling cited a 1997 article published by DNA expert Dr. Karl-Hanz Wurzinger that said one in every 13,000 reported paternity cases involving twins have different fathers.

It's called heteropaternal superfecundation and is common for dogs and cats. The phenomenon's incidence among humans is somewhat greater than the reported cases indicate because of the relative rarity of paternity tests.