I believe that countries and borders exist as a descriptive fact about the world. But I also believe that it's impossible to square immigration control regimes with any robust notion of freedom and liberty, and also that there isn't room for much disagreement on this specific point once you understand the intrinsic nature of such regimes. See Kukathas, Immigration and Freedom for more.

I know most people (of any nationality, not just Americans) don't understand this, but you and I are not most people. I believe things because it is correct to believe them, not because most people believe them (or not).

edit: to be clear, you can say "immigration control lessens freedom, but I'm willing to give up freedom—both yours and mine—because I value certain things more". That is a reasonable point, though I might as a follow-up probe into the motivation behind that. "I think the government gets to stop you from living with your spouse, but I'm okay with that because XYZ" better have a pretty robust XYZ behind it!