I heard a saying once on another forum and for years it stuck with me. (I'm going to paraphrase it and I might muck it up, but here goes!)
"Swinging is like a fire to a marriage. If the marriage is secure and trustful, swinging adds a nice warm flame to the relationship. If the marriage is laced with insecurities and mistrust, swinging will burn it out faster than a forest fire out of control."
Hey, I didn't muck it up after all!
As a side-comment, I've seen marriages where one or the other is doing something without the knowledge of their spouse (or plans to). I always recognize that as a bad sign in the marriage and I'll drop them as a prospect or playmate.
Trust and open communication = good.
Scheming and potential drama = bad.