If you DO currently have a sexual partner, then you're going to need to start taking control now, and wean them off their expectation of sex with you entirely. You ARE going to need to do it gradually, though.
Start out by not initiating sex with them anymore. When they initiate, start turning them down every second time, telling them you’re a bit sore “down there” and suggesting instead that you would like to see them masturbate for you.