Panel 1: Later still, we find John and Elly in bed. Since the Steve/Anne mess is keeping her up, she asks John why a man cheats on his wife. He sets himself up to get pummeled by asking what makes anyone cheat on anyone.
Panel 2: He then reiterates that he has no idea before listing motives that make sense to him: boredom, frustration and a need to test one's own attractiveness. A smarter person than Elly would have picked up on the fact that he does know exactly what would drive Steve to do what he did but her, she's got to be spoon-fed.
Panel 3: He then says entirely the wrong thing when he says that everyone is tempted to cheat but one must draw the line at not giving in. Realizing that he's just admitted that if he weren't lazy, he'd have acted on his impulse to have sex with her boss makes her shocked and angered.
Panel 4: Since John didn't offer up a comforting lie about how he'd never be tempted by anything more attractive, we find him after getting whomped by a pillow telling himself that some discussions should never be had in bed.
Summary: Yet again, we're reminded that Lynn is right to be paranoid about Rod because if he had no problems with her cheating on Doug, he wouldn't have a problem cheating on her.