Ok this is starting to look like a bug to me. Had one Sim in 3 consecutive pop-ups ask to be BFF, then Boyfriend/Girlfriend, then proposed marriage.
Awhile later, while on a date with another Sim, a different Sim that I know is already married, came up and wanted to be BF/GF, then after starting the date with someone else, that Sim proposed marriage. I just accepted to see what would happen.
What happened is, that Sim gave a hug, the date with the other Sim ended in 'bronze' instantly and the sim that proposed marriage became an 'unknown' un-met Sim. Clicking on the Sim that I accepted the instant marriage proposal had the options for 'introductions'. So I did a friendly introduction and the green bar appeared as if meeting that person for the first time. I did check before that in the relations and that sim was completely gone until I initiated an introduction.