Marrying a man who can provide security was expected. A man moving a woman into his house, having a baby with her while his wife was still around, and then marrying afterward was not.

I think it’s well-made that we, as an audience, are shown Peggy’s home life only moments before Marian sees it. Thus, we know the mistake that she’s about to make because the show had upended our privileged, preconceived notions only moments before.