It's not that I don't want to be there, but having done it both ways, I have come to understand the benefits of not being there. With my ex-wives, I was there. It was hot, and I'm glad I had that experience. My wife won't entertain the idea of me being there, so I've never seen her do the deed, and it's hot in a whole different way. I know that she's giving herself to him in a way she could not if she had her husband there as an audience and distraction. He also will not perform in the same way. I find it hot knowing that she's really fucking him, in a very intimate way.
The intensity of arousal starts from the time I watch her begin readying herself for her date, fussing over what to wear, packing things in a bag to take with. It escalates as she walks out the door and drives away. As the hours tick by, my thoughts drift from "she's going to fuck him" to "by this time she has probably (and later certainly) begun fucking him. Hours and hours of intense pleasure, yearning for her return.
Then another flavor of intense excitement as I see her walk in the door. That look on her face. The carefully coiffed hair with which she departed now slightly disheveled, The smell and taste of sex on her body, the cum accumulated in her panties when she peels them off.
She reveals the details in small increments over the days or even weeks to follow, each time generating a flood of adrenaline and excitement, leading to incredible sex between us later. We may drive past the hotel where they fucked, and she'll point it out. Or we may be out shopping, and out of the blue, she'll be reminded of something, and she'll whisper or quietly tell me some hot detail.
Even though I want to be there and to see and experience it first hand, I know that it would change the experience in a way that I think would take away some of the intensity I enjoy, and may even alter the way I see her. There is something to be said for the mystery. We want to see women naked, but if they all walked around naked all the time it wouldn't be the same. So I have to say, I'm glad my wife won't permit it. I think it's better this way. YMMV