Its rough isn't it
Poor kids - I was lucky not to have any when I split from my exh. I am full of admiration for the way you have kept it together for both of them, although with your ex being such an arse it was never an option not to keep it together was it!
As to what makes them do it - I have no idea. What planet they move onto inhabit is anyone's guess.
I think its the guilt thing. Also they have to spend so much of their waking hours keeping the GF happy that they forget the important things like - Oh! I've got 2 children and a wife already!
I wonder whether they mentally move out a long time before they even confess to the affair, so that they've done a lot of their sorting out in their mind way before you even know what is going on. I was as shocked as you are that someone who I loved dearly and thought loved me could act so dispicably towards me. Some of the things he said during that period still makes my hair stand on end!
Anyway hope you enjoy your DS's play tonight - don't forget its his loss not yours! (Unfortunately though it is also your poor DS's loss)!