Maybe they cheat, maybe they don't.
Even if they don't cheat they will always be wondering if the grass is greener elsewhere. After a while they convince themselves it is (even if it isn't true). Sometimes that leads them to break off the marriage and move on.
Maybe if couples would let each other sample the grass on the other side of the fence now and then it wouldn't seem so inviting over there (it seldom is!). Then the temptation wouldn't be as strong and they would be happier where they are.
We are only into the Hotwife & cuckold life now, but we have done some swinging and have been with other couples before. It was fun for awhile, but it led us to where we are today.
My husband never was into the swinging side of things. We swung with six or seven couples in our first year. I enjoyed most of the guys I was with and my husband seemed to be okay with his experiences, but most of the time he would end up watching me which sometimes meant the other woman wasn't getting all of his attention (sometimes NONE of it!).
Eventually we gave up swinging with couples and we found that the Hotwife life was best for us. I have no complaints of course as I still get to play the field. My husband seems content in his cuckold role with me having sex with others and coming home to him. He doesn't seem to have any desire to be with other women and never brings up the couples we were with or any of the women.
I guess he had his fill of sampling the grass on the other side, and decided that it wasn't greener over there. Now he's happy grazing right here at home. As I found out, he even prefers to do his grazing after another bull has spent some time in his pasture!
Lucky me! :o)