Yeah, the desire for variety will always be there. Men and women alike. I think having children effects womens stability that way more than a lot of men. Just an assumption of course.

Sometimes people end up having mismatched sex drives, or one is more kinkier than the other. If they cant work it out, or one or the other refuses to even try, than people may seek out someone who does to be their long-term affair partner.

Third reason is feeling taken for granted, knowing one is unappreciated by their partner...then comes along someone who does.

Case by case, who knows if cheating is a "two-way street" or not.

I definitely believe there are people who can be monogamous til they die, why not?