The idea is that "marriage" becomes a church thing, or a social thing.The idea was very good, sure; in fact, it was *almost* exactly correct. But only almost: It gets the last final little detail, namely who gets to define English, and who gets to abandon it, ass-backwards.
Marriage is *already* "a social thing". (Well, what isn't? Most everything is.) The thing is, though, since the Enlightenment, "social things" have been regulated by civil society. No, I'm not saying the weren't regulated before; they've always been. (That's pretty much inherent in the term, "social", isn't it?) I'm only saying that before, in the Dark Ages, "social things" were regulated either by autocratic fiat or by dogmatic superstition, and since the Enlightenment, they've been brought into the light of public scrutiny and (more-or-less) democratically-wrought, rationally-based, civil(ised!) Law.
Abdicating the power to define the very language your laws are written in, the language which more or less *is* your whole society, to the religious -- that is to roll all that progress right back into the Dark Ages.
(Sheesh, it makes one shiver with fear of the dark that you guys can't see that, it's so obvious. But maybe I shouldn't be surprised: It's the "boil a frog by putting him in the kettle while it's cold" phenomenon. Your United States have been slowly but inexorably religiofied for so long, you have a hard time seeing how close they are to finally becoming the United States of Christian Talebania.)