The post that mentioned the couple that got stuck sleeping on the floor, in bunk beds, etc. reminded me of our family vacation to DH's aunt and uncle's cottage. We were pressured to go, because, of course, FAAAAMILY! even though we all live within 60 miles of each other and see each other every holiday. So we go, and rooms are divided in a way that hubby and I are spending two nights on an air mattress in the hallway. Festive! Did I mention that everyone in that family seems to have pea-sized bladders? And there was one bathroom? And our air mattress was in front of it?
When the next year rolled around, we basically said "We don't think we can make it." The "selling point" then was--"It's on a lake, it's so relaxing, you can kayak, and swim ...!"
DH and I live on a lake. Year-round. We know what it's like.
I think in a lot of cases, the parents either still think of their adult children as minors, whose choices are to be determined by the parents. Or, they still see the family pecking order as it was when the children were young and the children's comfort is secondary to their own It's not necessarily malicious. They' just haven't adjusted to the idea that the adult children are grown ups with spouses of their own. They think, "Well, you're younger than me, you can sleep on the floor without your back hurting the next morning." There may also be a certain level of, "Well, we put up with sleeping on an air mattress in my parents' floor when we were newlyweds, now it's your turn."
Or at some level, they're not entirely comfortable with the idea that their adult children are sleeping with their significant others under the same roof and possibly playing scrabble. Putting the young couple on an air mattress in a common area of the house would definitely put a damper on that activity.
For my own parents, they were so used to enforcing boundaries to keep teenage DH and myself within the bounds of appropriate behavior that it was difficult to get out of that mode. DH and I started dating
when we were in our early teens. They were used to spend a lot of time with DH, camping, vacationing, traveling together. And in each case, they made the expectations very clear, even after we got engaged (around age 20) - no sleeping in the same room/tent until we were married.
So the first time we visited my parents house after we got married, DH was carrying in the bags and I said, "Honey we're in the front bedroom by the stairs." My dad's snapped, "You are NOT!" And we all sort of stopped and stared at him. He blushed and said, "Oh, right, you're married. Sorry."