They will inevitably reply in the negative... they'll say that the US is no better than, and in many ways worse than, most of the nations of the first world.
Then follow up with another question: "So do you believe, then, that the US should handle its borders like most other nations handle theirs?"
If they've a glimmer of intelligence, this question will leave them gawping and stammering, because they know what will happen if you try to walk over the border into Mexico and start demanding all the perks of citizenship (i.e., you'll get arrested and deported muy pronto).
With others, you'll have to explain it to them.
You could also ask the liberal “If America is such a horrible place what kind of monster would want to bring children from other countries here?”