Panel 1: Later that day, Elly tells John that Anne thinks that Steve is seeing another woman. Since Steve is pretty much a stranger to him and also since this affects his own life not at all, John makes the mistake of blurting out an honest reaction: "Really? Son of a gun!"
Panel 2: Elly mistakes this for his admiring Steve and also trivializing the situation which is why she's doing the palms-out thing when she repeats back his "Son of a gun" and asks him if he thinks that a man cheating on his wife is trivial if that's all he can say.
This is probably because she wanted him to whine "Poor Anne" or growl "Let's go over there and kill that fucker Steve" or something.
Panel 3: He angers her by saying that of course it's not trivial but is simply surprised that he'd take on the responsibility of another relationship. While this pisses me off as much it does Elly, it doesn't surprise me much. That's because unlike her, I remember that he finds it hard to deal with human relationships because he's a spoiled brat who sulks and rages when he doesn't get his own way all the time.
Panel 4: As she storms off in a rage because she just got reminded that John is kind of terrible at being a social animal, we see him thought-bubble "Heaven knows one's enough work as it is!"
Summary: What a catch, eh, people? He's just told her that the only reason he doesn't cheat is that it's an awful lot like work and he's going to do so again tomorrow. Since she's stupid, she doesn't really pick up on that and since she's gutless, she wouldn't do anything about it even if she did.