If I may be so bold as to offer a radical idea....
Pretty much all of the responses to this thread's underlying question are implicitly
prefaced with the qualifier: "In some cases....."
It's a truism that every relationship is different. I happen to think that's a lot of romanticist
horse-hockey, but even if there are really only, say, 250 different types of potentially-marriable
couple, that still leaves a lot of different reasons to get married, to refrain from getting married,
or to do whichever than regret it later.