Panel 1: As Anne gets ready to head home, Elly tells her that while she can't know for sure if Steve is cheating, if he is, he's crazy.
Panel 2: After all, they've built a home together and have three children. Since there's thus too much at stake, their marriage cannot break up.
It should be noted at this juncture that Elly's belief in the absolute sanctity of marriage ends up being contingent on sympathizing with the person being cheated on. If someone is an evil, mercenary and jealous Franco-Ontarian who gets in the way of Elly being presented with albino grand-babies, Therese can jolly well fuck off and die of the polka-dotted plague because she has no rights a WASP need respect.
Panel 3: When Anne shoots herself in the foot and gives Elly an excuse to treat her like crap by saying that maybe he needed romance and flattery, Elly sets up the punchline by saying that if he's seeing someone else, she's no match for you.
Panel 4: Elly is confused as Hell when Anne asks "Is that it, Elly? Am I facing a competition for my own husband?"
Summary: Elly's confusion stems from being Lynn and thus unable to understand how normal human beings think. A normal person would wonder the same thing Anne does but Lynn can't understand that and condemns her for doing so.
We see this again in a few weeks time when, having heard Martha explain to his dumb ass that she was only faking liking Lawrence to make him jealous, that pea-brain Mike dismisses her story sight unseen because he's too fucking dense to understand that she has free will and can do stuff like that.