There are three basic functions of a government.
a.1) defense of the nation's territory from external enemies
a.2) tracking of who "owns" what land in that territory
b) tracking who is responsible for minor children
In God We Trust
Historically the Church has taken over b). One solution to this is invent something like marriage, such that all kids from a given woman are "assigned" to some man, instead of just the woman herself.
In short, all kinds of complicated rules, so that if there is a question of "to whom does this minor child belong? who's responsible?", the Church has some answer, and it doesn't really matter what.
One advantage to handing marriage over to the Church, a separate entity to the Government, is that all sorts of crazy shaming memes and rules can be spun without impacting the implementation of the territorial functions of the Government. Rules to do with things like the "mysterious bond" between men and women, sex and relationships, childbearing and childrearing, tend to be more irrational and superstitious than mere Real Estate (that's my observation).
There's probably more need to adapt "marriage" to cultural environment, language, and so on, then with Real Estate. Land doesn't die. If enough or the wrong people die, or become indisposed, then children are orphaned. Children getting orphaned is really more important than, say, women getting widowed, although the Church and society may make it seem like the same thing.