(Strip Number 4584, Original Publication Date, 19 March 1986)
Panel 1: A short while later, an angry John asks Elly what possessed her to make that horribly embarrassing comment to Irene.
Panel 2: Elly defends herself by asking him why Ted proposed to Connie if he was in love with Irene.
Panel 3: This clearly means that if Connie had married him, Ted and Irene would likely have had an affair.
Panel 4: John scowls as he tells her that while this might be the case, it's no affair of hers.
Summary: The reason that he told her to step off is, of course, that he realizes that passive disapproval of a situation does not mean that he would want Connie to be cheated on. This distinction is lost on Elly and Lynn because they're kind of immature.