I don't understand. Why would he feel the need to drug her if they'd been having an affair? It's obvious she was willing before, why would he have to resort to subterfuge?
Because rapists don’t rape for sex, they rape for power. It gives them a thrill to take that power away from the women. It’s the same reason he paid her and kicked her out of the apartment. He wasn’t turned on by the vanilla sex anymore, so he wanted to make her feel used. Which he did. And then he did it again when he raped her.