***The question is -"where do we draw the line. "***

IMO, "the line" is up to the singer or composer of the song. Having said that, they don't get a carte blanche when they perform it. Some integrity and a bit of common sense should be adopted with regards to who is in your audience at the time. As the saying goes, "there's a time and place for everything".   
As a counterbalance, there's also the John Lydgate quote made famous by Abe Lincoln,"... you can't please all of the people all of the time" (truncated).
That's why you should be your own censor. There'll always be one Moaning Minnie no matter what you sing. Stuff 'em.