Oh, and you picked up on his needing to maintain a dirty boy persona.

A dirty boy persona is an artifact of societies that make sex appear dirty, via a variety of tools, the biggest of which is prohibitionism of any kind. That creates the phenomenon of "forbidden fruit". Since you already have passwords to all of this accounts, it does not seem to the case that prohibitionism comes from you. So he is playing out something that was instilled into him via the way he grew up, and completely independently of you. Since you are not the cause of his need to have forbidden fruit, there is very little YOU can do - but you can try approaching this problem in a paradoxical way, by radically removing the factor of forbidden fruit. It means that you will actually encourage him to text sex-related messages to those women. Set a measurable goal - at least 5 texts a day. See if he can keep it up. It will be an experiment - he might get bored. I am not sure he will, because the habit is old and unrelated to you, and his need to keep up his persona is deeply rooted in his relationship to the society around him, and not you, but at least you will have exhausted your own limited remedies. And, it might be fun to try this paradoxical approach (which would I am sure beat prohibitionist tactics in effectiveness).

If you do not like that approach, just drop the case - you have clearly considered the issue and do not see yourself being harmed by his behavior. Why fight harmless habits, right? Had you found yourself feeling hurt, then sure, there might have been a case for taking measures that restrict him in order to relieve your hurt feelings. But you do not have hurt feelings - you are reacting to the idea that his behavior might be inappropriate. You do not FEEL hurt. You just KNOW that in a model relationship, the guy does not send dirty texts. So, change your idea of what a model relationship is - it is easier to change your idea than to change his habit. Plus, since there is kink/fetish element present, you should realize that you ALREADY aren't living out the idea of a model relationship - so why not accept that as well? Who cares?