I think that you certainly have a potentially very interesting thesis here, but, it seems to me, the word in your thesis which might potentially lead you into difficulties is the word "expand."
There are various forms of power and control in the novel. Nurse Ratched exerts an insidious psychological control over the patients on the ward, and I would argue that she uses gender stereotypes and the men's sexual insecurities to consolidate rather than expand upon that control. For example, she makes Harding feel ashamed of his femininity and of his implied homosexuality, and then she uses his shame to consolidate her control over him. She also infantalizes Billy and makes him feel like a naughty child who has brought shame to his mother. When McMurphy experiences Nurse Ratched's group sessions on the ward, he astutely labels her a "ball-cutter." He seems to understand that she maintains control on the ward primarily by taking from the men their masculinity.
McMurphy has power over the patients insofar as he has the power to liberate them from their insecurities, their cowardice, and, in short, from Nurse Ratched's control. One of the most powerful weapons McMurphy employs to this end is sex. Indeed, Billy Bibbit's (albeit short-lived) transformation from an excessively meek, ashamed, suicidal boy to a confident, proud, and happy man is largely because McMurphy introduces him to Candy. Immediately after Billy and Candy have sex in the seclusion room, he, Billy, is at his most confident and his most happy.
However, I would argue that McMurphy does not use sex here to expand upon his power but rather to reduce or relinquish it. He wants Billy to be able to exercise power over himself and to relinquish his dependency upon others. He ultimately does not want to have any power over Billy but rather wants Billy to be his own man. Therefore, I would suggest that McMurphy uses sex as a means of transferring, rather than expanding upon, the power he exercises over the patients.
Bearing these points in mind, I would perhaps alter slightly the wording of your thesis. Perhaps your statement could instead read: "Ken Kesey presents sex and gender as integral to and synonymous with notions of power and control." I think this revised thesis statement would open up the possible links you might make between sex and gender on the one hand and power and control on the other.