Clark has to observe Bruce during a mission that involves Bruce sleeping with someone. Maybe the sex was always part of the mission, maybe it's Bruce improvising; maybe Bruce knows Clark is observing, maybe he doesn't. Either way, Clark is uncomfortable ... but he's also really into it. Because, holy shit, he's into Bruce.

Something something Bruce/Clark. Maybe Clark starts acting weird later and Bruce has to pry it out of him? Maybe Bruce indicates that he's fantasizing about Clark during the sex, either to rile Clark or because he assumes Clark will never know? BOTH? Something else? I don't know!