I don't agree with, but do understand, why churches will refuse to sanctify gay marriage. The Church ultimately decides what's right for its individual practitioners.
But I don't understand why they feel gay marriage must not happen full stop and why it supposedly undermines heterosexual marriage? The catholic church does not agree with pre-marital sex but it isn't out there trying to stop the rest of us.
I genuinely would like to understand why some religious groups think same sex marriage harms their heterosexual relationships. If simply makes no sense to me.