I haven't had the pleasure yet but from the list I would choose number 2,there would have to be intimacy between them, without it sex means nothing to me so it would definitely be the kiss.
You're assuming he's a Cuckold, maybe he doesn't want to be involved in the fun ,maybe he knows she would shut it down if she discovered he knew.
I'm sure he's completely secure about her commitment to him.
Maybe he doesn't want her to know about his discovery, maybe he's just thrilled by the knowledge she is getting the shit fucked out of her on a regular basis.