How does sexuality play a role in the story?

As the story opens and Frances and Mike walk along, their conversation reveals that this married couple has been in bed together on Saturday night into late Sunday morning. Frances notices Mike looking at another woman and proposes they spend the day together instead of in the company of others. In the midst of her proposal and just after Frances has kissed his ear suggestively in public, Mike looks closely at another woman and notices that "she looked very solid and strong and her belly was flat, like a boy's, under her skirt, and her hips swung boldly because she was a dancer and also because she knew Michael was looking at her." The tacit exchange between Mike and the woman is purely physical, frankly sexual, and entirely fleeting.
Eventually, Mike and Frances speak honestly about his wandering eye. Mike admits "I can't help but look at them. I can't help but want them." He even confesses that he may one day act on his interest. Mike acknowledges that his objectification of women is based purely on their looks and his sexual desires, and this moment of devastating honesty does nothing to temper his appetite nor clear the air in a positive way with Frances.