There is a continuum: someone, somewhere called it a sheath, and the metaphor was so apt that it was repeated until it eclipsed whatever word was current usage before. Neither you nor anyone else is qualified to judge which words are metaphorical and which aren't.
> Neither you nor anyone else is qualified to judge which words are metaphorical
This statement refutes your attempt to claim particular words are metaphorical.
Well, you're right in that vagina literally means vagina in English. And yes, there are occasions where language is unambiguously metaphorical. I do not, however, appreciate how either of these undermine my larger claim. Not sure if you disagree with me on a more fundamental level or not.