The book was originally published in 1928. The social mores around society, position and women sexuality were firmly in place.

Off the top of my head, I can think of a dozen ways that the MC interest alone would be "shameful." Women were not expected to enjoy, seek out or even desire sex.

She was having sex with her grounds keeper, someone that would have been viewed as lesser than her. She was his better socially. She then gave into him taking the lead, giving commands, being in control. ALL of that would have been shameful.

The simple act of having sex with a skilled lover would have been a sensual experience.