Male A: "Dude, some chick tagged you in a picture where you're making out with her friend, untag yourself before your girlfriend see's that!"
Male B: *Reads text* "Looks like she already has, no more worrying about untagging myself anymore!"
Female A: "Oh my gosh, this creepy guy I talked to when I was drunk took pictures of us and tagged me a few times, I hope people don't think him and I are actually friends!"
Female B: "Girl you need to untag yourself, you don't want any association with that!"
When a relationship status turns sour and one (sometimes both) of the individuals untag them self from pictures with the old boyfriend/girlfriend.
I'll hang out with your sucky friends, but if you guys start talking Battlestar Galactica again I'm going to have to untag myself.