[The Fuse]

What you told them sounds like common sense to me. I'm sure that part of their reaction stems from simply being worked up about the idea of having sex with other people in a context that makes it okay. They see something that looks really good, and perhaps they aren't thinking clearly. Horniness has a pesky quality of doing that to people (at least to me). They apparently don't realize yet that if they are insecure in their relationship, having sex with others isn't going to help! Maybe you should ask the husband how he would feel if and when he saw his wife really enjoying having sex with another man, and vice versa for the lady. Maybe that image would help them realize what they are considering.