Well, first off, they don't have anyone else as the "break up" was my doing. But apart from the chain, not even going back to being "girlfriend" comes up in the romance.
I read at another forum that there needs to go some time before the sim can get married again. For instance, with the travel needing 2 days. But again, the only thing I have is the little sign at the Sims picture with 2 broken rings. Other than that, I don't see why they should not get back together just after the break up, even.