I think this one hits a several buttons, all at once, for me.
I mean, right off the bat there is the "getting away with something naughty" part. All sorts of things push this button, but this is DEFINITELY one of them.
Then, there is the voyeuristic button. I mean, watching is good, and watching someone I have a connection to is even better.
Then there is the projection part. Since I know what he's feeling, I can imagine the sensations, and of course that's good.
Then there is the vicarious part, of enjoying her enjoying herself (it would be no fun if she didn't).
Then there is the anticipation part, because I'm going to definitely enjoy my part.
Then there is the participation aspect, in that I get to be part of her kink too. Knowing that after sucking, she'll be dripping wet for me or whoever else is next is a huge turn on.
Finally, although it doesn't get to control me, there is always that primal tingle of jealousy. Even if I'm thoroughly enjoying the entire spectacle, there's still that little burn at the base of the skull, where the lizard brain still whispers.
So yeah, it hits all the buttons.
I think the reason that oral does it more than plain old sex is that it's usually slower and more deliberate. By the time the fucking starts, everyone is purely in their moment (I hope), and so it's not so much shared or deliberate.