But it's a fallacy because the -supposed- naturalness of something doesn't say anything about its righteousness. Petrol is natural, but highly poisonous, and clothes are unnatural but pretty necessary. And the call to nature argument can be pretty dangerous since it's often used in favor of pseudomedical practices. And without talking about the hypocrisy of the argument when it's used against queer people, since the people using it doesn't have any problem form many unnatural things.