I feel as you do some times, it hurts when you feel the W has moved on already. I think TH post above is pretty accurate, As we during the meltdown dont know how bad it really is until the bomb, the future WA is checking out of the M before us. My W moved very quickly as well, Bomb early july, served D papers early august, and she moved out August 20 something.
I also have young kids, I think it is too early to introduce the OM to your S. I think whem the WA moves quickly they feel the need for some form of complete closure and that it will feel more free quicker. But again as TH says when they move this quick they dont really have the sense of what the long term effects will do. I know I would not introduce someone I date for a month to my kids, no frigin way.
I agree on the background check, when W and I separated 3 years ago and she had OM, I did it and also later found out during our 6 month split, she did not have OM around the kids when she had them. She did at least not disrespect me the fact I am their father no one else. Do you know if OM has any kids?
