One thing about this particular robot . . . she probably in some sense has free will . . . but she also has certain drives very deliberately built in. She probably can't not want sex. Which is, OK, pretty much true for the rest of us except in our case evolution built it in, it wasn't anybody's fault.
It's somewhat like the main dish in The Restaurant at the End of the Universe, a cow-like being who solves the problem of humane eating by having been bred to actively want to be eaten, and after talking up the deliciousness of various portions of his anatomy, excuses himself to go commit suicide. "Don't worry, sir, I'll be very humane." The problems here rather pale by comparison
All that said, I think this isn't really the venue for deep ethical exploration. It's a romantic comedy, and the ultimate answer is going to be, look, SHE thinks she loves you, SHE thinks she wants sex, and that is in the end not less real for having been built in, so why not give her what she wants? I'm not saying that's actually a decisive argument, although it's fairly defensible. But it's the line a romantic comedy has to take, overtly or tacitly.
That doesn't address whether it was OK to build her like that in the first place, but in terms of their relationship it's a little late--it's no more ethical to forcibly change her mindset now that she's been made than it was to make her. Which leaves either dealing with her as she is, or junking her, which doesn't seem very nice either.