I almost wanna say there might be a translation error? Bc the dude is saying he'll do a marriage ceremony, but not actually get married? But then as her yaps, it's like, it sounds like common law marriage???
Marriage via church is a religious thing, very performative. But after the ceremony they are still expected to sign papers to confirm via the law that they actually are married (at least, I think thats how the US does it).
Hence why divorces are possible in modern day. Divorce thru law.
So that also means common law marriages (that only need a document signed) give married couples hella benefits, including stuff involved with taxes, home buying, financial assistance with children, and more.
So I'm p sure Kotanin is aiming for a performative marriage, not legal marriage (๑.๑)
Idk, hopefully there's a more concrete explanation
Frankly, my views are probably too progressive to understand why common law marriage is worse than traditional marriage in this manga. That said, the plot twist was funny af lol