People change over time, and the people who made that vow at the age of 20 are probably not going to be the same people at the age of 40. If all goes well, the people that they have changed into will remain compatible with each other. But, sadly, all does not always go well. I know multiple older people who are trapped in miserable, loveless, emotionally destructive marriages that they feel they cannot leave solely because their 20 year-old selves made that vow before God. It's not that these people didn't "mean it", it's that they (like all of us) could not perfectly predict the future.