Sooo quick update:
Luckily this Sim has the Serial Romantic aspiration, so she had several boyfriends and girlfriends lined up. I dumped the first guy and tried marrying two more people and I'm getting the same message. The weird thing is that the option will randomly be available for a minute and then if I click it, the action will cancel and it will be greyed out again. And I can't seem to notice any specific pattern with when the option becomes available. With the first fiance, I even tried adding him to the family and employing him. Still got the same message. And when I added him, his marriage option to marry the girl was there, but it would just immediately cancel every time I clicked it.