Good honesty I wasn't expecting
My other partner is worried that your and my relationship is unhealthy. I don't want to hurt you and make you sad. I trust you to tell me if I ever overstep any boundary of yours (even if you realise it later). I don't want you to let me push you places you don't want to go.
I play with you because you let me. I play with you when I'm in a playful mood and you appear receptive. If you don't appear to want to play, I don't play. If I don't want to play, I don't play. If you want the game to stop, say so
I got the following reply from the other man.
I am not sad. I don't mind the playing but I am not able to make out relationship into one like you currently have with your other partner *hugs* I am not poly. Perhaps one day that will change, but where I am now I've never had any desire to be that way. And if I was to join in with you lot right now it wouldn't be because its what I want, it'd be to keep you happy, and that wouldn't be healthy at all.
SO I don't mind the playing, but it definitely has a limit which at this point in time I am not going to cross.
So I told him
Thankyou for your honesty with me.
I don't want you to do anything that you would be uncomfortable with and that you consider unhealthy.
I'm not going to push you into doing something you think might make me happy, I'm not going to expect or ask you to do anything I feel you are uncomfortable with. Your happiness is important to me. Your friendship is important to me. Both of these things are more important to me than any potential other relationship that we might possibly have (especially if that relationship is something you don't want or need).
He was worried that he had upset me by telling me this, but I really have moved on. It is the most he's said about this and it shows me that he has thought about it, and I can't ask for much more than that.