Hello, I am on level 6 and I got the propose icon, I used cupcakes to skip my goals. If that doesn't work, do all the goals you can until it comes up. Also, many people asked how the sims move in to each others house, what I did was; ( By the way, I am the highest level of partners) I took all of the furniture out of my house and the I got the sim to go to her/his partners house and I placed all the furniture down. Then I went to the town map and went to the now vacant house. Now click and hold it ( I use my IPad 2 ) It will bring up a couple options: it brought up my sim, relocate sim, relocate house, and demolish house. Choose demolish house and it will disappear. Since your sim will have no where to go, it will stay at his/her partners house.
I hope this helps you, if not, I know much more things you can do so, reply to this comment, Good Luck!!