There is a "it", _that's the whole point of the movie_!
The twist is when you realize that it wasn't a woman (which you can easily forget by how 'human' it acts), but a heartless robot doing everything in its power to obtain what it wanted.
"and even the author tried to weasel out by saying that the robot doesn't actually have a sex"
There's like 5 minutes where the protagonist discuss with the builder about why he gendered the robots. The authors were clearly aware they could have made a non-gendered robot.
(Now, the builder was enjoying them sexually, but that's a character in a story, not the authors.)
Of course, the ultimate need for a gendered robot is create the set for the story.
1. To get us, the audience, to be able to relate to it. Nobody is going to care if you format a super-toaster hard drives. They will if the toaster looks and act like a human being.
2. To enable the protagonist to be even more emotionally involved with the robot.
"and this says more about what the authors think of robots and female sexuality than anything else."
It says they know how to write a story, not much more.