The premise wasn't original, but so what, the film still did a great job exploring the concept of AI.
The scene where Nathan discusses his Jackson Pollock is a metaphor for his creation of Ava. Like the painter, Nathan is compelled to create the AI, although he has no clear understanding of what its purpose will be. Had he known the outcome from the beginning, he would have never started. Understanding the consequences is secondary to the recognition he will receive as the inventor of the most significant technological development in history.
He makes the robot sexual because he wants the robot to succeed in coercing Caleb, thus passing his version of the Turing Test, because in doing so it means he has succeeded.
The scene where Nathan discusses his Jackson Pollock is a metaphor for his creation of Ava. Like the painter, Nathan is compelled to create the AI, although he has no clear understanding of what its purpose will be. Had he known the outcome from the beginning, he would have never started. Understanding the consequences is secondary to the recognition he will receive as the inventor of the most significant technological development in history.
He makes the robot sexual because he wants the robot to succeed in coercing Caleb, thus passing his version of the Turing Test, because in doing so it means he has succeeded.