What does that have to do with religion?
If 2 sides agree to abide by a civil contract then so be it. Who gives a shit if they are both in agreement?
Who has jurisdiction over that? A US court, other courts or both? How is jurisdiction determined? Who determines the jurisdiction? If they agree to some model and set of rules and laws then that has a course.
Where is religion in any of that?
One sentence eliminates religion and that is the challenge in this case specifically.
So at least you agree that a civil contract could contain clauses referencing international or religious law.
Let's say person A accepts the jurisdiction of a NY court.
Let's say person A says in his/her testament that they want their assets divided in the manner described by Sharia law X (for example, 2/3 to the boys, 1/3 to the girls).
Person A dies.
Boy 1, Boy 2 and Girl 1 have a dispute over the distribution of the estate.
What do you do now? Does the NY court ignore the testament of person A? Does the NY court apply Sharia law X? Does the NY court refuse to apply all the portions of the testament that are directly linked to Sharia law? Does the NY court refuse to apply all the portions of the testament that are even inspired by Sharia law?
Civil courts resolve civil disputes. Parties can contract to use many laws, including religious or international law. When a dispute arises and they want to go to the courts, you have to decide what you will do with their agreement.
If you really are interested about the issue, these are two interesting posts about it:
The Volokh Conspiracy American Court Refuses to Honor Lebanese Islamic Court Child Custody Order
In that case, a MA court refused to respect a Lebanese religious court decision.
On the other hand, read this one:
The Volokh Conspiracy Islamic Law in the American Legal System
There, you could see some of the nasty effects of simply disregarding any type of religious law in the American court system.
I think there needs to be a balance here. Sharia or other rules that go against the public order (for example, if person A and B sign a marriage contract in country C that allows A or B to kill the other if the other commits adultery) should not be enforced/allowed, while there are other decisions or laws that should be looked and potentially followed at by courts.
I know the right loves the black and white type of argument (NO RELIGION!), but black and white arguments always fail in real life.