Yeah, the only way Sharia law would be able to operate in the United States is in the same way that other religious law is allowed to operate - by private contract in cases where it doesn't involve criminal conduct or other conduct that is against public policy. Which, you know, seems right given that whole First Amendment thing.
I'm fine taking issue with the use of religious law at all in solving legal disputes, but to single out Sharia is silly and discriminatory. It reduces your argument essentially to "our religious law is better than your religious law," which isn't a position the government is allowed to take