I think you said it wasn't the discussing that you wanted it was relief from imagining them together... is that right?
The way I have dealt with this in terms of sex parties that my friends have and now don't bother inviting me to
is that I imagine them all fucking each other. Different people coupling up, helping each other, what they must be like having sex, what their private parts look like... I walk right through it. Perhaps it is better than actually being there, because I go through all of it in my mind and can shut it off when I want.
It's like what I do with jealousy, I take the time and space to really get into the worst case scenario so that nothing phases me... I allow myself to go into all the detail and bring myself out the other side so that I see that there is another side... in terms of my polyfuckery friends, we will all go to karaoke in a few weeks, we will all update our status on fb, we will all go to work monday morning... nothing really changes other than they have more connection to each other and I don't. They have a shared experience and I don't... I walk myself through what I think has happened for them as a result and how I fit in with that too.
You are not invested as much as RC to this man but he is your friend... perhaps making the decision to invest less in that friendship would be helpful.. at least for the time being anyways. This seems to work for me anyways. When I struggle with what some people do I chose to pull away and invest elsewhere. Besides, metamour relationships are different from friendships and perhaps backing off from friendship once he intimately knows your wife would help. I know it makes me feel threatened when I know someone has been intimate with someone I have... I feel uncomfortable and like we are all in the know now, yet I don't know what you do, and you don't know what I do and that is just awkward and makes it all less special for a time.
After sex, it might be an idea to make sure he doesn't expect a pat on the back and we are right back to being pals thing happening... it might take a long time to be close to him again if perhaps ever again... that happens and is completely valid and okay. It is a good idea I think to make him aware that that might happen for you and what you need from him in that regard. ie, back off. It might happen with RC too and that is also okay. You don't have to be okay with this and be all glowy and compersiony right away. You are allowed to react and be emotional I think... it will bring up stuff that you don't even know is there yet, so respect that and take it as it comes.