A preference for unbiased discussion of controversial issues is not itself uncontroversial

Exactly.

Once my students started a discussion about religion, so for the sake of discussion I told them that this discussion is allowed to continue only if both of them agree to switch their roles and defend the other position best they can.

The religious student said that this manner of discussion would be interesting generally, with a different topic; but specifically when speaking about religion, defending the other position, even jokingly, would be a sin or at least something dangerously close to a sin.

I had no good answer to that; mostly because I realized that even if I could come with some clever explanation that would convince the student, there is a good chance their parents would see it otherwise, and I did not wish this kind of a conflict.