Jon was staying with his mother because she broke her ankle and her husband had to go out of town. While there, yes Jon went to a bar, where some college girls recognized him and asked to have their pic made with hm. They then made up the story about him partying with them so they could sell the pics to the gossip rags.
Jon is having problems, but not marriage problems. He is having trouble dealing with not having a personal life anymore and he isn't enjoying working from home anymore either. He wants to take a break from the show after season 5, and go back to work. He isn't certain that he wants to do a season 6. Kate wants to come back for season 6 after they take a break.
As I said on another question, psychiatrists say that the most stressful things in life are marriage, death of a loved one, birth of a child, changing jobs and moving.
In 8 yrs they have gone through:
marriage
birth of 8 kids the death of Jon's dad
moving 3 times
Jon got laid of during Kate's pregnancy with the 6, and 2 more job changes after that. Kate's from full time nursing, to part time nursing, to working on the show, writing her books etc.
ETA: going to a sports bar to watch a ball game does not equal cheating on your spouse.