I completely understand the feeling of dumbfounded, angry bewilderment, WTBH. I don't think it's dumb to let it upset you. I think it's a natural response to seeing the level of deception an abusive partner uses in his approach to all relationships. And you have every right to feel hurt and as you process it all.
I've been thinking about this and its impact on DS a lot. Honestly, I've been trying really hard to stop beating myself up repeatedly on the choice I made for father for DS. But, I try to remember that it's not my fault that AXH is the way he is. I never would have thought for even a minute that he would be the type of parent he's ended up being. He let me believe we were completely on the same page as far as raising DS and how involved we both were. I was astounded at the 180 he seemed to do.
Just because I understand that AXH isn't capable of being, or willing to be (?), the dad DS needs and deserves, doesn't mean I can't be hurt by that. Especially when I see how it affects DS.