One morning I walked in on a flat-mate fucking his girlfriend, he was going hard at her in missionary and she smiled at me over his shoulder and said 'hello!' - he was quite embarrassed but didn't stop fucking though! All very innocent, I didn't know she'd been sleeping over. I apologized and left quickly.
Very similarly, a friend walked in on me while I was staying in his house for the weekend; think he did it on purpose as my girlfriend was being noisy. I was doing her in mish too, and when he walked in, I pulled out lost my erection. My girlfriend stayed exactly as she was, legs bent and spread, pussy open and wet, and talked to my friend casually until he left. Think she was hoping he'd join in, maybe he was too but it didn't happen. Would have thought the men would have been OK in this situation but the women embarrassed, but it was the opposite.
At swinger parties it's the rule that if the bedroom door is shut, those inside want privacy. If the door is open they're happy for others to come and watch or even join them. Most people leave doors open.