Why doesn't Holden have sex with the prostitute?

Earlier in the book Holden described himself as "yellow". Then we learned that he cared about those who couldn't take care of themselves, espeacially the ducks. He wanted to know that even when he couldn't see them that they'd be okay and survive. The same is aplicable to the prostitute. He described her as looking no older than he is. At sixteen, this girl had to become a prostitute, had to do horrible things for unknown reasons. We do not learn why she had to do what she did, but we do learn that she never wanted to hurt Holden, prove when Maurice came to collect the money and Sunny stopped him from seriously injuring Holden. Holden could not sleep with Sunny because even after she told him she worked every night, she is still a girl, she is young and childish, but Holden wants toprtect her.