Hmmm...this article about Exeter SU suspending their Christian Union for being exclusive is interesting. The CU has counteracted by taking them to court. RHUL kicked their CU out years ago for doing the same thing and it didn't cause this much fuss. *boggles*
Hopefully the courts rule for the SU, as the whole point of SU societies is to be for all members of the SU and it seems to be common for CUs to enforce a "Christian only" policy.