In years past, they didn't have to. Now the government can go after the father for money the government paid to support his kids...they can also go after the father for past payments made to the mother..
..all those that don't like SS blame the government that made it mandatory. Not the people that were forced to pay FICA. Yell to congress not to us old fogies that had to pay for decades into a program that took the money and redistributed to their friends for buying votes...
That is responsibilities without rights, or as the Founding Fathers put it, taxation without representation.