I have this too from time to time. I had a Sim who had a college friend. When he aged up, she called him for a date. They went out. She asked to move in almost immediately. Romance bar was almost all the way up, then she asked to move in. Two days after they moved in, he autonomously wanted to propose. I have a video that I did which shows this, if you want the link. It didn't bother me that much, but I've been seeing this happen with a few friends too on their Lets Plays and such. I wasn't concerned that much because I just rolled with it, like I normally do.
If this isn't a bug, can the relationship progressing be tuned down just a tad to give us the ability to do "slow burns" if we want? This seems to be more prevalent with the latest update.