How is the theme of sexuality and repression dealt with in the chapter? (chapter 16-18)

In Chapter Seventeen, sexuality is again a tactic of McMurphy. He questions Nurse Ratched about her breasts. The theme is continued in some sense by Chief Bromden later when he wonders about the birthmarks of the Catholic nurse in relation to the desire for purity. His observations about the Catholic nurse suggest the detrimental effects of sexual repression; unlike the tightly corseted Nurse Ratched, this nurse seems to demonstrate intense guilt and shame about her sexuality. The narrator describes this situation almost entirely in metaphorical terms of "stains," with obvious sexual connotations.

Although McMurphy becomes more bold and authoritative in this chapter, Nurse Ratched remains calm and reassured. She has regained composure because she knows she has control over the situation in the long run. She can determine what happens to McMurphy and whether or not he is ever released from the asylum, so she can tolerate any short-term challenges to her power, even if occasionally he can draw others to his cause.