On natural moral law conceptions, is it allowable to chew gum? Or perhaps, under what circumstances is it allowable to chew gum?

How does one differentiate chewing gum, from say forcing yourself to vomit up food (so that one can continue eating, or so that one will not become fat)? How does one differentiate it from masturbation?

If we see masturbation as sin, perhaps even as grave sin, then why is chewing gum not likewise sinful? Where does the relative "importance" in the moral evaluation derive from?