To clarify: the Supreme Court did not say that assistance could not be restricted to single parents. The "Man in the House" rule said that a woman was not single if she even had a boyfriend who was not her children's father, if her boyfriend ever visited her house. That's the part that was struck down.

As for the restriction to single parents, I can certainly see the potential for unintended consequences. Paying people not to get married means fewer people will get married — just as paying people not to work, as the "welfare cliff" effectively does, means fewer people will work. I broadly support the idea of helping people who need help, but it needs to be done in such a way that it doesn't tend to trap them in their situations.