oh, maybe I wasn't clear about what I meant when I said I knew I had stayed in a relationship that I knew wasn't good.
No, I have never been in a relationship like the one I have with him before. What I was meaning to refer to is that in my past, I have been in a monogamous relationship with someone (unrelated to this situation) well past the point where it was working. It was hard and unpleasant, and we probably should have broken up before we did. But I think people stay in them because they have lessons to learn, and you can't leave them until you are ready to leave on your own terms. I mentioned it because I can see why he stays, and why I don't wish to pressure him to leave, even if it would make them both happier.
No, I get no thrill of deception, nor do I consider this "safe". I much prefer depth, but am under no illusions that my relationship with him now has depth.