For some odd reason, the "Propose Marriage" interaction isn't coming up between my playable female ghost (Adult) and Michael Bachelor (Adult). They are girlfriend/boyfriend, so I can't see the reason that would block the interaction. My only guess is that this occurs because she is a ghost, not a Sim like Michael. Maybe ghosts just cannot marry? Have you had any experiences with this?
I'd like to note that they met long after she became a playable ghost, and it wasn't me that made them go steady, it was Story Progression. They have a child together, too, again, via Story Progression.