So they were in a relationship at Uni ? Then I would say, while the game displays them as not being in a relationship and also treats them as such, part of the game still knows that they are.
Have you tried going back to Uni with them to see if they are still in a relationship there ?
If it is like this, I don't think there's much you can do short of using a mod to fix (which we aren't really allowed to talk about here). They will probably need to be together without being in a relationship.
If you don't have any further issues, I would just play on without them getting married or having a relationship.
