Perhaps part of his thrill is her cheating on him
From reading the thread in its entirety,I'd say all of his thrill is her cheating on him.
I know because I have the exact same fantasy,I have no desire to sit in a corner watching her with another guy for the mere fact I know she wouldn't be able to let herself fully enjoy herself and let herself go in my presence.
It's all about her enjoyment and pleasure and my presence would surely be a barrier to that ,so for her just to go out ,have a great night, dance the night away and go to a hotel or wherever and fuck the guy senseless who she has been attracted to all night is the ultimate fantasy for me.
Watching her get dressed in a sexy outfit, putting on her lipstick, mascara, mosituriser etc would have me throbbing,watching her come home the next day looking all dishevelled, mascara and lipstick all smudged, hair all messed up, stockings laddered, dried cum stains in her hair for bonus points would probably give me a hands free orgasm.