Back in the present, Shelly is criticizing Lyman for being afraid to depict his grandparents' "sex life" (4.6.6). Um...GROSS.
Although he's writing a novel, not a nonfiction book, Lyman has no interest in delving into sexual issues. He blames Shelly's concerns on her generation's obsession with free love.
In Lyman's mind, sexuality was a private thing for his grandma and her generation, so he should respect their wishes and keep it private as well.
Shelly says that this strategy will make readers think that Lyman is purposely avoiding the issue, which—to be honest—he is.
But, that sex scene didn't actually lead to a happy resolution. Oliver found out the next day that he would have to travel a bunch if he were to join the survey, which means that Susan would have to go back to Milton.
Instead, Oliver decides to become the manager of a mine in Leadville.