Dad wants to know

Was there some other guy who petitioned to be acknowledged as the father? That is the only way I can see you not being the "assumed" father. DNA is only part of the equation as many fathers do not share DNA with their children due to adoption or fertility treatments. Now, if you both lied and said there were no children born during the marriage, this kid wouldn't be part of the divorce settlement, and court wouldn't be any the wiser.