This is one of my favourite topics, and here is how I was first introduced to it.
The first couple of times we hung out with him, my wife and her bull just played lightly together (kissing, fingering ect) and afterwards he always slept on the couch. The second time we saw him, I got the impression that she wanted to share a bed with him that night, but they hadn't fucked yet and I was sure if they shared a room he would fuck her while I was asleep and I would miss out on seeing their first time together, so I insuated back to her that it worried and upset me too much.
Then the first time they planned to actually fuck, we got a two bedroom suite (ostensibly so he could have a proper bed since sticking him on the couch repeatedly was starting to feel mean).
I had no idea what they were cooking up until halfway through the evening (just after I had watched him fuck her for the first time) when he asked me, while she was in the toilet, what the ...... arrangements would be. I thought it was strange because it felt like it went without saying that I would ...... in bed with my wife and he would take the other room. I tried to play it cool and I said "we'll ask her what she wants to do" thinking that she would definitely choose me.
When she came out he asked her, and she looked apprehensively at me and said "would you be ok with it if he sleeps in there with me and you take the other room?"
I was shocked. I felt my heart break a little bit. I didn't know what to say. Also, I could tell from both of their demeanours that they had discussed/planned this beforehand.
Again, I tried to be cool and said I was fine with it. I went outside for a ciggerette and when I came back in they were making out again on the couch. They stopped when they saw me coming and whispered something to each other. Then straight away my wife declared that she was tired and suggested we call it a night. They stood up together, she kissed me on the cheek, smirked at me and said "goodnight". He shook my hand and said "tonight's been really fun dude. ...... well yeah?". Then he took her by the hand and walked her to their bedroom. I saw him take off his shirt and pants just before he turned around, smiled/nodded at me and shut the door.