To be honest, this problem lies with metadata and file naming first, and agents second.
Consider that you would have to name your media in a consistent manner, and have a metadata library that will consistently match the data correctly. There currently is no consistent method of naming adult media, nor is there an easily-accessible metadata library that collects all this data for matching purposes.
You also have to take into account the multiple Actor/Actress naming over studios vs amateur content, genre conflicts, web scenes vs physical releases vs web series and you have a pretty tangled metadata mess.
A little-known fact is that The Movie Database does, in fact, cater for adult movies (you enable it by ticking the “Allow Adult Media” checkbox when you select your agent for a library) but the contents are few and far between (do you volunteer to update the database? It doesn’t happen automatically). But TMDb restricts its library to movies, so web-series and scenes are not allowed (and how would they be named for matching, anyway?)
So there are agents for some of the various databases out there (all quite incomplete, and all skirting the litigious studios or don’t allow reasonable API access).
Plex created the environment that allows for custom Agents to be made, and they have. That the agents aren’t consistent is not Plex’s fault. The taxonomy just doesn’t exist yet.
