The qualitative difference with the 17 and 24-year relationships is that they're ongoing. The implication is that some people pair for life, with the wedding no

The qualitative difference with the 17 and 24-year relationships is that they're ongoing. The implication is that some people pair for life, with the wedding not being the critical factor, while someone with a string of few-year relationships might have some quite a different view on relationships, a drifter of sorts. (Or, they might just be unlucky). That 'drifter' would be strongly inconvenienced by forming and dissolving marriages every few years. So one way to indicate by the 5-year stage you are not that way inclined and that you are committed to your current relationship is to tie the knot. The signal only works one way - the two of you having chosen not to marry doesn't imply anything (in the strict logical sense).