deriving pleasure from it. Or when the person giving it stops deriving pleasure from it. If there is no trust. If it is being done to someone who never consented to the act being done on them. If the intent or motive is to destroy flesh or emotional balance and not create bliss, a safe-haven and/or a moment the two people can exist in that transcends themselves. If either of you says no more and someone continues and that sort of thing isn't already well hashed out within your dynamic. If it is being used to coerce something unwillingly from one of the partners. Its all about intent. stuff like that.