No sin at all.

I usually like sleeping out under the stars, or under a tarp, but the last trip we took my wife and I used a Tarptent Double Rainbow. We had massive thunderstorms, and the feeling inside the tent was snug and warm and very nice.

I think there is a strong innate feeling in humans for a small confined space with walls all around. Call it a back-to-the-womb thing if you like, or maybe a cave-man thing, but it goes back to the earliest humans and it's hard-wired in our brains.