[NSFW] Grinding definition and connected tags (with visual examples)

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I like tags
There is a tag on Derpibooru called “grinding” with this desctiption:  
To rub one’s crotch against an object or another person, such as their partner’s leg, a male’s penis, a female’s vulva or table corner, to the enjoyment of both the rub-er and the rub-ee. Distinct from tribadism and hotdogging.
 
In my opinion grinding on Furbooru should be phrased differently so that it would be more clear and also include males.
 
Short description:  
Act of rubbing one’s genitals against body of another character or against an object.
Detailed description:
Examples: vagina against a penis (but without penetration), penis against balls, penis or vagina against a pillow
Note that this tag should not be used for:
  • tribadism - when two or more characters are rubbing their vulvas together
  • frotting - when two or more characters are rubbing their penises together
 
By the Derpibooru definition it should also not be used for hotdogging, but then why exactly? I can’t find a reason. All non-penetrative rubbing except rubbing of the same genitals (vulva-vulva and penis-penis) could be just tagged with grinding, and then some of them with hotdogging as well, where applicable.
 
Visual examples
 
This would be just grinding:
 

 
This would be grinding and pillow humping (there is a ^tag named that on Derpi^ and it clearly is used for both males and females):
 

 
This would be grinding and intercrural sex:
 

 
And this could possibly be grinding and hotdogging:
 

 
This might be ~~grinding~~ too as it’s not yet ~~anal~~ because there is no ~~penetration~~ (so just imminent anal ) but there is contact of genitals with body:  
>>1723  
(Edit: As for the thing above, I don’t think it should be grinding in this form as the penis is just clearly about to be inserted into the anus, but grinding would be just rubbing the anus with penis without the intent to actually put it inside so this image is not a good example of that.)
 
This would not be grinding because its tribadism or frotting:
 

 
But then there also are pictures like this:  

 
It could be said that this would not be grinding because balls/penis is just placed on someone’s face, no indication of rubbing, but it’s not like movement can be seen if it’s just a static picture, so should this be grinding?
 
And then this? It even kind of looks like rubbing, so it would technically count.  

 
There are no relevant search results for “grinding, penis on face” on Derpibooru except one single animation where there is clearly rubbing
 
If images of genitals in contact with face are not intended to be tagged as grinding then the description could be modified to include:  
Act of rubbing one’s genitals against body of another character (except for the head) or against an object.
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