Your argument is better than most, GS, but it still has fatal shortcomings.

TWU doesn't deny entry to ANYONE. Everyone is welcome, regardless of their religion, sexual orientation, etc. Those who enter voluntarily agree to abide by certain rules. LSUC, however, DOES deny entry to certain people: those who got their law degrees at Trinity Western.

I can certainly accept that LSUC may establish rules for its membership, just as TWU does. For example, LSUC could state explicitly that their members must offer professional services to anyone regardless of the sexual orientation. Voila! Problem solved, no? 

However that's not the approach LSUC has taken. Rather, it seeks to exert influence over a religious institution by denying entry to those who are or were members of it.