I slept in "their" bed about 3 times. She suggested it because she was out of the house working a night shift and their bed is bigger and more comfortable than the futon in the spare bedroom. (He's a tall guy, so that made the futon extra uncomfortable!) It was okay the first couple of times, but not the third time. His and my relationship was going downhill by then. I didn't feel like I was getting enough time with him. (They live together. I lived a little over an hour away and was lucky if I saw him once a week or slept with him once every two weeks.) As I laid in their bed.....it was a reminder to me of how she and he sleep together night after night....wake up to each other morning after morning. And how much I wanted more times like that with him.
If she was home at night, I would never sleep in their bed and displace her. That was HER bed, not mine. He would come and sleep on the futon with me. We'd make do even if it was a bit uncomfortable. (When I moved to be closer to them and start a new job I lived in their house for the first month until I found a place of my own to rent. He traded off sleeping every other night with one of us, in general, with a few exceptions. Unless she was at work, we slept on the futon in "my" room.) He wanted me to come into their bedroom when they were in bed in the morning to tell him "good morning" and give him a "good bye" kiss before I left for work. She was fine with this. (If their bedroom door was shut, I wouldn't go in...although that only happened once.) In fact, she'd make it a point to tell me to have a good day when I came in to tell him "Good morning". I miss them.