The unsaid tension between them, over commitment, devotion to each other - why not spend Sunday alone together? - and the fate of their marriage. I get the sense of people, or at least Michael, who is not really ready to commit fully, and the negotiated peace that Frances accepts.
Not the point of the story itself (which IMHO is to explore real humanity, not give pat, perfect answers), but I don't understand other people:
If something will make your partner happy, why not do it? Imagine how overjoyed Frances would feel, how special she would feel, if Michael responded - 'hey, if it bothers you, it won't happen again', and maybe, with a wicked smile 'but I'm going to need to look at you a lot more.'
And imagine how not respecting her feelings makes her feel - and it's every effing time you leave the house together, for the rest of your lives. Is it really worth it to cop some brief stares?
Is there anything better in the world than seeing your partner happy? Even omitting the feeling of seeing that joy, purely selfishly, what would make Michael's life better. I guess, in my reading, he's not ready to try it and find out.
(And any general statement must make many assumptions about the relationship. I think every relationship is unique and they cover an inconceivably - I mean, we really can't conceive of them all - wide range. It's all a bespoke arrangement between two people. Some are open, some closed, some long distance, some together every moment for decades, some passionate, some practical, some angry or sad or joyful or whatever. IME we almost shouldn't be using this same word for each one; it's like saying everything with a processor is a 'computer'.)