No mods--- But I did discover the cause of the problem. In short, it comes from screwing around with the age of the NPC in the couple. Both times this has happened to me I've realized it was because the NPC was a teen who aged up into an adult. I then aged them down since my own sim was still a teen. I aged them back up with my own sim, which probably confused the game and created the bug... I'm not going to be doing that again.
Still haven't found the solution to the problem if it happens, though.
