I don't have wifi and managed to use my paperwhite 2 unregistered for years, though I did have it 'registered' by Amazon in order to download their books and sideload them. However you can't use the Collections feature unless the device itself is registered.
An unregistered device also loses the ability to use other features such as Vocabulary Builder.
I recently borrowed a friend's wifi and registered my kindle in order to use Collections.
I suggest that you go ahead and register your kindle, then keep it in aeroplane mode. As long as you don't buy books from Amazon, they will have no way of knowing that you're reading terrible porn.
Why are you worried about Amazon knowing what you're reading? I'm sure they have better things to do. Maybe wearing an aluminium hat would help
