I have lived long enough to understand that every individual is unique and different and, therefore, every marriage is unique and different. It would appear that many couples engage in the swinging lifestyle. The rules you have set down appear to be very similar to those. I have read in other sources. The only issue that remains is one of genuine consent. I looked at two parts of your post: “He’s a 35-year-old engineer, a bit hesitant, always the more cautious of the two. She’s a 32-year-old graphic designer, eager for adventure, always the one pushing their relationship towards new experiences…Before entering, they stop. John looks at Mary, his eyes full of unasked questions. Mary squeezes his hand, conveying confidence…” Only those two know how they feel. But can we be sure he genuinely consented? Or did he yield to her desires, believing that would be the only way to prevent an affair? Perhaps he takes the marital vows more seriously than she does. It is one thing to engage in this behavior if both sides truly want it. I am sure many do. But opening up any relationship holds the possibility of endangering it as well as enhancing it.