Most married people know about precisely one marriage, their own. And some people in these marriages really haven’t put their shoulder into their own marriages, and so they know surprisingly little about even their own marriage.
You keep repeating this, that at BEST a husband or wife only knows about their own marriage. That’s a simplistic statement that is fundamentally false. Most married people know quite well at least two marriages: their own and that of their parents.What really upsets people is to realize that counter to the secular/dissenter myth, most priests know more about marriage than nearly any married person, who only knows - at best - about their own marriage.
In fact bad habits and bad attitudes of a husband or wife is often picked up from what they saw in their own parents’ behavior.
Also — depending on the circumstances — a married person might know well the circumstances of other people’s marriages too: their aunts or uncles, their siblings marriage, and the marriages of their very close friends.
That said, you do make a good point that most parish priest probably have a far wider perspective on marriage for all your reasons you’ve already stated. So you shouldn’t weaken your point by repeating a basically false statement, that a married person only knows one marriage well: their own. Most people know well the marriage of their parents too (for better or for worse), and some even more than that.