As you say, these situations are not "cut and dried" and that is the case for much of what we experience as human beings. Most things fall outside of the black and white areas and into the uncomfortable gray. People often use the fact that there are gray areas as arguments against voluntaryism and the NAP while failing to acknowledge that there will always be gray areas regardless of what "system", or lack of one, is in place.
RE: Is Fraud--and by Extension, Marital Infidelity--a Violation of the NAP?