So I discovered that duo is aliased to couple, even though the wiki, even before I edited it today, has for a while now said that it's for romantically engaged pairs, not just any old pair. I'm willing to bet that a big reason so many images are tagged couple when they shouldn't be, is this alias right here.
Duo was suggested for use over in the other thread anyway, so I think we should go ahead and do this. Otherwise a lot of what I'm doing is going to be undone, I think.
Aww crap, I accidentally put this in General. Sorry gaiz :-/
Updated by Granberia