Have a level 9 loving relationship sim move in/get married?

Hello! I have an issue and would love a real answer! I will give all the information that I have: I have 4 playable adult sims in my house. 2 of them are happily married to each other. My 4th sim is in a level 9 'friends with benefits' relationship with another character. I have never gotten a 'move in' or 'propose' option. I would like to know the reason. Also, the other sim in question is retired, not sure if that matters. (Not sure why it would unless a bug?) Can you not get other sims married if you already have 4 living in your house? Even though that sim isn't one I can control? If so, man that sucks. Anyway, would appreciate hearing from someone who knows the answer. Can't find any info online about it.