According to the wiki for crotch_rub, it's to be used when an object is involved and done by the person themselves for the purposes of masturbating (post #386060 as an example). I've seen it used sometimes where the grinding tag would be applicable (post #447094 as an example).
For this, I propse we implicate crotch_rub -> masturbation, since the wiki defines it as basically a way of masturbating.
The wiki also leads me to think that posts where one person is rubbing against another (things like post #439300 or post #225485) should be tagged with the seldom-used frottage tag instead of crotch_rub as well, and we could also implicate grinding -> frottage, though that wouldn't be a big deal if it weren't.
There's also a bit of tag clean-up and some wiki updates that I'll take care of if this goes through.
I'm all for better wiki clarification, especially since there's noise brewing in the Comments section on what is or isn't frottage. So far, danbooru has several different "rubbing" tags:
frottage: sexual rubbing that doesn't fit under one of the tags above (crotch rubbing against someone's arm, leg, tail, etc.)
That's all the tags I can think of right now, and they should get implemented into the individual wikis under a "See also" section if a decision is made. Let me know if I'm missing one.
As for the implications, I'm leaning towards a no on both; for crotch_rub -> masturbation, there can be images where the crotch can be rubbing against an object, yet give no indication that the character is masturbating (post #358752, post #321442), and for grinding -> frottage, I think it's important to keep the tags distinct.
+1 Hillside Moose. I've gone through crotch_rub -masturbation from time to time in the past, trying to weed out mistagging, and there have been some questionable cases.
I actually mostly made the crotch rub wiki page to conform with the original use of the tag millennia ago - iirc it was mostly populated around when an influx of photos was uploaded (all now purged, of course). It seems that the intuitive usage of the tag includes girls having their crotch rubbed by someone else, grinding, frottage, etc., at least by looking at tagging habits of random people. Hopefully nobody minds that we keep it the way I set it out in the wiki, but I concede that it could be up for debate.
