This is the view I’ve often had when a husband is first allowed to witness his wife’s coupling with me. I don’t know what this guy was doing, but his pattern of twitching repeatedly and frequently is one I’ve seen many times in cuckold’s watching their wife’s coupling. Go ahead now… watch.
Notice that he’s fully erect, yet periodically pulsing in arousal as his eyes witness a moment his penis responds automatically to. It could be an expression on her face, it could be their eyes meeting or perhaps the sight of her snug around my cock. Though I typically don’t want a cuckold spurting until he’s told to, this event clearly demonstrates to his wife that he can never claim to not enjoy being her cuckold and also proves that the cuckold is unable to maintain control of his own penis without our intervention.