Leave religion out of it. The position I'd take: "Is it ethical to force your (anyone's) beliefs on another individual in our society?". Don't let them go down the "committing a crime" road. The supreme court says it's not a crime. Give other examples, the freedoms our country affords, free speech, free assembly, free press, free choice, civil rights. Throw in free religion is you like.
Being told you're delusional does not necessarily mean you're mental.