1) Yes, the dissolution of the relationship, not necessarily that of the marriage.
2) Even with them being together, there's a difference between married and non-married couples. Married people live together, unmarried people don't have to but they can. Unmarried couples can take time away from each other without it being too serious, whereas for a married couple one would be expecting divorce right around the corner. Someone can try and tempt someone in an unmarried relationship without coming across as a total homewrecker. It's easier for an unmarried couple to keep secrets from each other. There's less pressure for an unmarried couple to take that next step and have children (I do not like giving Superman a kid). And on and on, the relationships are different even when the two are together, and there's a lot of interest in an unmarried Clark and Lois we don't get to see (how long did they really *actually* date officially before marriage?). And that's not even to mention that some writer's could more easily write around Lois if they suck at writing her character but otherwise are great Superman writers, while other writers can come in and still make great use of her and her dynamic with Clark.
My point is, there's a lot people can like about then not being married, for various reasons. They don't have to be unmarried, but they've been unmarried for so much of their history that making them married all the time just feels like an unnecessary step.