Hi there!
I know it's years after the discussion but I literally made an account just to tell everyone the solution I found which might help someone else in the future. No mods, no cheats required.
I faced the exact problem and I tried so much to fix it, spending rl hours.
Basically I realized it had to be some sort of glitch so to fix it I had my Sims break up. Might seem a bit bad but I try to role-play it into some romance cliche of "separation and climactic return" sort of stuff.
Anyway, after that I console, apologize and basically heal the relationship, before returning to romance. As soon as they were hitting it off again, the options confess attraction, take it steady, propose and marriage all came in one by one, one after the other Instantly.
This might not work guaranteed for everyone, but it worked for me so it's a chance right? Anyway good luck everyone.