Hm no, it is not only teens. This household are also young adults, roommates, and can become bf/gf perfectly fine after fulfilling the friendship/romance bar. With the current Sims I play, there is no possibility for this, they are single ever since they aged up from being teens, it says they are lovers (still single though) and can only propose.
Try using the "Ask to be friends" friendly option. That will remove all the romantic status as well as gf/bf tags to where they are just friends again as that option does not affect the friendship bar. Then build the romantic relationship again to see if that brings back the "Ask to be gf/bf" option. Don't save over your existing save . If you need to save, just do a Save As to create a separate save for testing until you know if it works or not. If it does, then you're good and should see the "Propose" option again. Then use that save to continue on. If it doesn't, you won't lose anything but some time testing it and can go back to your existing save.
Hope this helps.
