Honestly if I was in a relationship and for whatever reason gained weight to the point where my partner found me unattractive I'd be insulted if they didn't tell me.  I wish more people had this attitude. Really, when you enter a relationship, you're agreeing to be with that person as they are now, plus normal wear, tear and aging (time ruins even the prettiest of faces, unfortunately). If you undergo a massive personality shift, go insane, or double or triple your mass, then you are no longer the person who originally entered that relationship.