It's really tough when I have my 9 year old daughter watching along - but maybe an educational opportunity. I explained that what David and MM are doing is wrong. However, I added that I don't think David really is married to Katherine. Still doesn't make it right, though. They should both steer clear of each other unless and until he is single. It is very hard not to root for them, though.
I keep wondering if Katherine won't grow a conscience and be the one to step up and admit to him that she's been playing along in an elaborate farce. Unless she really believes they are married.