She has sworn to remain at his side during his final quest to rid the world of Voldemort. She even attempted to explain the mysterious ways of girls to him when he was fifteen. Sex is, in fact, the characteristic that defines the difference between a platonic relationship and a romantic one. Harry needs a witch, no matter how good-looking, with whom he can laugh.
When he finally gets to be with Cho, they never have any fun. Even though he does respond to Cho, their relationship ultimately fails for a number of reasons, including grief and guilt over Cedric's death.