Why does Matt have an affair

As far as I understand it, there are several reasons for Matt to have an affair.

1. Because of the dignity of a man: Since Jules started up the company, he took the house work, such as taking care of daughter, esp. he need go to take part in the parties of her daughter, this makes that he can't avoid to meet with the mothers. From the mothers' attitude to Ben, we can imagine what it will be towards Matt. Naturally Matt's dignity will be influenced by them.

2. Matt's sense of achievement: housework can't give sense of achievement even to a woman, let alone for a man. But everyone need to proof his achievement. 

3. The pressure from Jules: as we know Jules is the kind of person requiring perfection. It's possible for her to give that to Matt some time. Plus she becomes the internet focus. These will bring a lot pressure to Matt.

4. The problem coming from their relationship: as Jules get busier and busier, they have fewer and fewer time to meet each other. This do terrific harm to their relationship.