just doesn't have it in him.
What he didn't have in him was doing it to your face. It seems that it was easier to do it behind your back - what that says about him? Well... a few things come to mind..him just being a coward is one.
But, I suppose it *could* be to do with his 'compartmentalising' during the affair ...when he was doing it behind your back he could be doing it to the image he had of you in his little 'justification box'...not the 'real' you he saw when he was in your 'real' presence. Doing it to the real you would have made it all the more 'real' to him and he was dealing in fantasies at the time. He could also have just believed that old saying ...what she doesn't know about won't hurt her.. so he thought that doing it to your face where you knew about it was worse...
Or he really was just a coward - who preferred to attack you behind your back and not to your face for his own safety...I don't know.
Why would he see the former as more hurtful and harder to inflict?
I think it would also be harder for him to inflict it to your face because then he would have to be braver and 'see' the hurt he caused you, feel guilt for it and also pay consequences for it. A whole different ballgame than moaning about you to OW who doesn't get hurt by it and (presumably) likes hearing it too.