Since you are married at this point, either don't do the "date" or kick your H to the curb.
If your H got back together with you strictly for you S's sake....there is nothing in it for you, IMPO....tell him not to let the door hit him on the way out. Don't put up with this behavior. He just gave you a glimps into your future, is that the future you want?
IMPO, you can feel free to go out with this xbf you mentioned, but you need to be free of your H first. Then you can be free to find someone who does want you for the awesome woman you are!