Hard to do when some orthodox groups accept divorce and remarriage and others don’t.
“and the hardest part of all, to tell their congregants to, in the future, have only church marriages, not to get secular marriage licenses.”
This is probably what is going to end up happening, either as a choice or as an involuntary punishment from the state—’discriminate by not renting your hall to state recognized married gays we won’t let your pastor issue state recognized marriage licenses for your members’ or even if you don’t marry gays at all.
Freegards
I though about the divorce angle, which is why the agreement has to be limited to the rules for marriage.
For example, though the Catholics prohibit divorce, this only applies to Catholics. They would still call married people from other churches in the agreement, married, if they chose, or unmarried. But if they wanted to become married under Catholic auspices, or for the Catholic church to recognize their existing marriage, the Catholic rules would apply in addition to the agreement rules.
Oddly enough, this would benefit all the churches that are part of the agreement, because many of their congregants who just got a secular marriage, would want to renew their vows in a “real church wedding”.