Hm. Well, on danbooru it seems like the vast majority of images tagged "masturbation" involve self-pleasure. Perhaps the wiki entry on the tag should reflect that? As far as I can tell, we already have tags that cover pretty much any form of masturbation other than self-pleasure (such as fingering, footjob, handjob, etc.).
I'm no huge Japanese expert, but it seems that the pixiv tag オナニー does follow this convention. Maybe that's just a difference in the meaning of the words, though.
