For the effect you are after, I would agree with Tojo about sash cord. It feels like a soft and gentle caress as it goes around you. (Although never heard about putting it in the washing machine before - will suggest that to the powers that be.)
I also have rope from the end of the spectrum used on me, albeit carefully I hasten to add. It is very course, hand made and bought in a Turkish market in the middle of nowhere, originally intended for tying up donkeys. The harshness of it adding a poignancy of it is own.
Nylon rope sounds particularly harsh - eeeeeeek.
But why do you not want marks? I love mine, they are an evocative reminder, and one can always dress to cover them up if you need to.
cariad
(going off into a happy trance thinking of rope marks and how they got there .....)