prosser states it as a privledge and the restatment does not list it as such an element for its torts, prosser was also a commentator on the restatement and his books rely heavily upon the restatement. so in these cases the books does not treat consent as an affirmitive defense, nor is it listed as rule 12(c) federal rules civil procedure (not his book). so in that sense it is just an defense, but does not explicitly state it must be raised to be heard. It's more of an issue for the jury to figure out, and not neccassarily a defense or element. the burden is not on the plaintiff, nor does the defense need to meet a proponderence standard to be met. it's more of a matter of fact than law by more understanding. consent is much more case specific and wide spread law would not properly address the ambiguitious in the law.