I don't think it's natural for two humans to share a bed. Or even a room, for that matter. As a kid, I shared a room with my sister. My two brothers shared a room, as well. We would dream of the day we each got our own room. And eventually, it happened: dad hired a contractor to put up a wall and split our toy room/school room into two bedrooms.
We each moved into our own tiny space. The only place in the house you could go for solitude; the only room you had the authority to kick someone out of (and that happened frequently, in my case). It doesn't make sense that we grow up and finally get our own rooms, just to grow up even more and devolve back to sharing one when we get married.
Steve and I had a queen-sized bed up until last year. Neither of us are any bigger than average (OK, admittedly I'm a tiny bit heavier than average but it's mostly in my chest), but between us and the dog, it always seemed crammed. I would smack Tucker with my foot and he, in turn, would growl at me. Or I would whap Steve with my arm. It was enough to make me want my own twin-sized bed again. I'm not a great sharer.
Then we got a king-sized bed. Now I have no idea if Steve is in bed with me or not. We could fit two other average-sized people between us. Tucker's got mad hops being able to jump up onto this monster. Embarrassing to admit, but we bought him those puppy stairs. He is much too prideful to use them though: he'd rather have bad legs in old age than be seen using that prissy contraption.
Steve and I have completely different bedtimes. I like to be on my side of the bed reading at 10:30 so I can fall asleep a couple chapters later. Steve comes up to bed around midnight: a drink in one hand and the remote in the other. This infuriates me. If I'm asleep, I don't want to be awoken. If I'm reading, I want to read without the blare of the TV. This is why I'm seriously considering getting my own room again. We've got extras. And I'm a creature who hates to devolve.