You really have my admiration, and sympathy. I'm new to this too, and it's one thing to find out your wife was sexually intimate with another man, it is something else entirely to know it is happening in advance, and then while it is happening. I've spent my whole life hearing "men just don't understand" this or that, and most often I agree. But this experience, at least at first, feels like the testerone in you is backing up, tightening your chest, then choking you. That pain, when it turns to rage, is what causes so many murders when a man unknowingly walks in on his woman with another man
I know, I know, after more experience, this is supposed to get easier, and it has for me. I also know that I hope to reach the point where I can feel compersion -- joy and love for my wife for the joy and love she received from her secondary. But at the beginning of this transformation in a relationship - ouch.
This is tough. And if I wasn't here with my wife, receiving her reassurance after her dates, I don't know how, or if, I could handle it.
Be strong. It gets better, I think. It can't get worse. And either way it's better than cheating.