I had this tonight, too. They kept getting wants to get engaged/married. I kept cancelling the wants then, when the "Become Soulmates" aspiration milestone wouldn't complete, I quit the game, cleared my cache, and repaired. When I went back in, they stopped getting those wants and the milestone was fulfilled.
I don't know if it makes a difference, but this was one of those whirlwind romances where they became boyfriend/girlfriend, got engaged, and eloped, all over the course of a couple of hours. The elopement was immediately after they got engaged. Maybe it was too fast.