I don’t think tags can be appropriated. If someone uses #erotica for a pair of tits off the web, we can’t force him to change tag.
But we could choose to use always the same tag when writing erotica (as first tag or one of the following). Like... well... #erotica seems the most obvious. The Italian user I’m in contact with users #erotic, so that’s what I use for my posts in Italian. But I use #erotica for the English posts already.