Do you know someone who is married with multiple partners purely for adult fun and entertainment? Have you ever wanted to do that yourself of feel you could get caught if your spouse was totally against it? This may or may not be the same thing as swinging, but you can look at this situation from your own personal opinion. Yes, some people like to have their cake and eat it too, but then, there are those who are open and willing to share their goods for some serious satisfaction.
I think this is common when you get to a certain age when the spark in your first love (or your wife any way) is gone, you tend to look for other excitement outside your marital bed. Yes, I know someone who is like this, he is married but have one or two other women on the side.