Hey K, your stbx is on the defensive. He will not say anything that he thinks will hinder his chances of getting back together. You expect him to say to you that he was planning on moving in with her? Or that she was pregnant? Or that she met your kids to see if they got along or whatever? How would that really make you feel? I am sure that you would not say, wow, stbx is being honest with me....maybe I should give us another chance.
I find it interesting that you think that he would still be leading a double life if you had not caught them.
I also think a one night stand is a mistake. A three year relationship is more than a mistake........