The OP clearly states that the other two consider themselves to be a 'unit'...it's pretty clear from the post that we are talking about a married couple +1 (longtime friend).

Look, to each his own, and I think I may have done this (or something close to it) in my mid/late twenties with old university friends, where one was a couple, but once you're a little bit more into your relationship, the last thing I'd want in the room is a third wheel. There's also the question of what goes on in the room, but that would derail this conversation into OMNI territory.