OD, I feel for you brother.. I went through the same feelings about this time last month when my W told me she was actually in the BF/GF phase with someone.. 2-3 dates in the month beforehand and then BF/GF.. No knowledge of each other before any of this..
I actually had a good laugh when the shock/anger/anxiety wore off not long after as it is a nearly textbook rebound R which is the same as yours sounds.. I still don't LIKE the thought of W doing this, but I have set similar boundaries of not knowing/wanting to know about their "R".. As DB states, we can choose to respect the W's life choices, but it doesn't mean we have to agree with them..
A funny side effect of all this is that it has done wonders to my detachment.. I suddenly look at W differently, and honestly DB stuff kicks in.. I have noticed I don't think about her anywhere near as much before BF BD, and I have "tuned" my interactions to nearly ALL DB principles.. I have also gone as much NC as possible, but as read on another persons post (HP or Mozza I think), when the time has come to reply etc I have been neighbourly, but have also tried to mirror W's interactions..
All in all, there is a light at the end of the tunnel.. It just means we have to work harder..
Me:35 W:31 S6 + S9 T: 10 years M: 7 years BD: 7/2014 S: 8/2014 W has new BF: 12/2014 Still fighting the good fight!!..