"LSUC could state explicitly that their members must offer professional services to anyone regardless of the sexual orientation. Voila! Problem solved, no?"

No, because the problem is not that lawyers in Ontario are practicing discrimination, which is illegal.

The problem is that TWU's covenant discriminates against LGBT students in particular, and anybody who doesn't adhere to evangelical religious views in general.  For LSUC to accredit them would create unfairness for those who aspire to practice law in Ontario --  evangelical students would have a slight advantage in the number of law school spots they can compete for overall and a big advantage in gaining admission to TWU in particular.