It seems the problem is with the Bachleor/Bachleorette partys.
I ended up restarting and speeding through a relationship with a new girl, when I did her bachleorette party, when she came home, the same thing. Only this time it said she was my fiancee under relationships but her partner was her best friend. (A female who toasted her at her bachleorette party).
Also family tree showed both my sim and his fiancee as single, but relationships didn't.
Weird, anyone else come across this bug? --- http://24.media.tumblr.com/tumblr_lt37rzo2Jk1r4d7qpo11_r2_400.gif