I'm not sure it was ever intended as a film song, I think it was always meant to be Ringo's song for the album, ie Side Two. In the end, of course, Act Naturally was used instead. The good thing about Beatles films is that the songs do not tell the story, not in the sense of other musicals of the day. For instance, Elvis might be driving along with a pretty girl beside him and suddenly he bursts into song, usually with a really bad film back-drop behind him!
Regarding the line "You're quite content to be bad", well, the next line kinda fits in the 'rich-girl' scenario: "with all the advantage you had over me".