Cheating is cheating and that is what he is now doing on this man. The explanation of why it isn't is bullshit to me. Cheating is lying to a lover about sleeping and being deeply involved with another. He is doing this and therefore cheating in my books.
It really is a tell tale sign of how much he cares and loves you and him if he can't be honest with him and wants to hide you. He doesn't sound very committed to the idea of either of you... just fucking perhaps? I wouldn't trust that there is any other intent really.
The foundations of poly, as far as I am concerned are integrity, honest communication, respect and empathy. To me, if a person does not hold true to one of these, then they are not ethically non-monogamous and not worth investing in. At least if this is a constant occurrence anyways. This man is not acting on any of these foundations and I would wonder if he is worth your time. you could do better.
I understand that he is concerned that he might lose this guy, but this guy will leave when he finds out, or at the very least blow up as he is being cheated on. Why would you want to bring that kind of thing on yourself? That just sucks. Would you want someone to do that to you? Does this man not deserve to be respected, to have honest communication, to have you both put yourself in his shoes and do you not want to act with integrity?
I think that rather than worrying about your jealousy you have bigger issues to think about with this. I think you should promote all sitting down together and talk it out. Come out with what is going on for you, listen to what is going on for this ex of yours and most of all, find out who this new guy is, get to know him, create a friendship with him and a metamour love that is honourable.
Then think of what makes you jealous. Chances are, after all of that, the issues will of changed and there will be other focuses.
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