I hate to be judgemental in a situation like this, particularly considering that Jon has parental responsibilities, so let me first say that Jon is a jerkoff if he's actually cheating on his wife.

But that woman is a shrill harpie who is never happy with anything, and routinely makes her displeasure known to everybody and anybody who is even remotely willing to listen.

Everybody's to blame here. Also, ugh.

What are Jon's options here? He can either:

a) remain faithful to Kate, who everybody will agree is a horrible woman, and be miserable with his life. If he is miserable, he will undoubtedly make life miserable for those closest to him (i.e. his kids)

b) leave Kate, which will undoubtedly be difficult, BUT will also be terribly upsetting to his children, and make being a responsible parent difficult (do you have any doubt that Kate would deny him visitation rights?)

or, c) stay with Kate, but get some much needed comfort and affection from other women. While it is de rigeur in this country to immediately assign blame to the unfaithful party, I see Jon as picking the least destructive and least douche-baggy option available to him.

All this is null and void if he is a bad father, in which case he should just leave and spare everybody the nonsense.