My sympathies, Zappafreak... I also agree with those who say it's not worth worrying about whether you're poly or not at the moment. It's time to regroup and take care of yourself and your child.
That said, and I hate to sound like a paranoid person, but please try to protect yourself if this does go into divorce proceedings. On other boards, I have read more than a handful of times about one spouse leaving the other for their new BF/GF, only to serve divorce papers accusing the other of adultery (and smearing their reputations as cheaters, if they weren't public about their poly relationship).
I recommend holding on to as much evidence as you can that this was a mutual arrangement. If it comes down to an accusation of adultery, you have something to fight back with.
Again, I'm extremely sorry you're going through this. Take some time for yourself and your child, and hang in there...