Alternative relationships can often be difficult to maintain. It's the same reason gay men and women sometimes stay in the closet their entire lives: Other people don't understand. Or maybe other people don't approve, or maybe other people feel torn up inside when they see someone who has come to terms with their own uniqueness, especially if that uniqueness isn't necessarily pretty.
I'm well aware that humans are curious and knowledge-seeking by nature. It's understandable for almost anyone to become curious after learning of a couple who are non-monogamous. But as citizens of an educated society and a well developed culture, we also know damn well that those intimate details are none of our business. We know that prying is a decision only a child would make--or an adult with the mind of a child. Unfortunately, America seems to be increasingly proficient in producing just that sort of adult.
Read all about the history of open marriage.
Read about Tilda Swinton's adventures in open relationships.