Normal? What is normal in the first place?

It is generally expected to stop once you hit puberty, and since that point there’s a general expectation that sleeping ^(literal^ ^meaning^ ^of^ ^the^ ^word)^ with someone else will be reserved to your partner.
But why though? It’s not like people have need for physical contact just until they hit puberty, and obviously not all physical contact is sexual. It’s not weird to hug, and it’s also not like it has a time limit.

Take this for example. Anything sexual between siblings is unnatural and not normal. Anything sexual between humans and animals is unnatural and not normal as well. But it is generally viewed as normal to sleep with your dog or a cat. What’s the difference? Why is such contact between different species normal, yet when it’s with the same specie, we arrive at this question.

I guess it depends on what is “normal”.