I don't know... I can't even find these things now that I'm looking for them to see what it's all about, so how easy would they be to just stumble over?
I do think they should be labelled, but to be perfectly honest, labelling erotica and putting it in a special category really makes life easy for the young, curious teenager, doesn't it?!
There are always two sides of everything.
I do understand your concerns about this kind of content (child porn) can trigger someone to live out the fantasy, but I doubt there is any more risk with these books than a large number of Criminal Minds episodes. Neither will "inspire" a sound person to commit horrible crimes, but both can inspire someone sick to live out their fantasy.
Speaking of stumbling over "surprising" content... This is what happened to me many years ago (last millenium...): I do machine embroidery and the software I used back then is called PE Design, often just referred to as PED. This was back in the early days of file sharing, and out of curiousity I put "PED" in the search field, and what came up was the most disgusting list I had ever seen! I saw the first few titles (along the lines of the OP book titles), screamed out, powered off the computer in panic (completely irrational!), and just stood staring at it before finally turning it back on making sure none of that crap was actually on my computer...