How much room is there to store this portable bed?
There are foldable memory foam mattresses that are easily stored, but still take up a bit of space. I've used one that was "MemorRest" brand and it was lovely, but I don't remember if it would comfortably sleep 2 adults - if you go that route, you may need 2 next to each other.
That's all I could think of that wouldn't rely on a frame or being inflated to keep it up.
Keith and I actually stopped having sex on our old bed because we were wearing down the frame...
Have you considered just changing the location of your sex so that the place you sleep survives?