Like many have said, there was a reason (I found the reason kind of cheap though) to why his wife and Arlene was so susceptible to the other guy (has been a while since I read it, don't really remember names). Once he understands why, he finds some peace with it. Up until then as far as I remember, he was having problems accepting it and more or less having some form of ptsd about it. In the beginning, he is considering divorce etc but when it comes down to it, he just doesn't want to give up on his family.
The problem I had with Arlene & Jeff that made me stop reading after the second book was that it became much more 'clan focused' rather than 'family focused' that it was in the beginning. Suddenly, instead of being about Jeffs family, you get entire chapters and POVs from side characters and people I would barely even consider side characters, such as some random construction worker. I wouldn't mind the whole clan development so much if it just had kept the POV on Jeff and his part in it, since from the beginning the story was about Jeff and his family.
Also, like Sunkuwan said, you get about 30 chapters of a new girls introduction. I remember really liking one character and was really disappointed when she married a side character instead, after I spent hours reading her story and getting to know her.
It's just my personal opinion, but I would have prefered it if they kept the side characters harem-building and sex life off screen and just give the side characters girls a short introduction from Jeffs POV when they meet and leave it at that. Just say something like "Jeff, this is X, we met her at Y, she does Z for a living." I really don't need to spend a fourth of one book learning the life history of a side characters love interest. Keep the long introductions and deep characters for the main character that you have related to from the start and actually care about, you know?