First things first: we're almost positive that "Sunny" isn't this girl's real name. She's the prostitute the elevator man pimps out to Holden, and when we first meet her we're all excited that we're finally going to figure out why this book has been banned and re-banned. Instead …
Nothing. He never has sex with her; all they do is talk. (Okay, she does take off her dress—but we're pretty sure she's still wearing a slip or something.) By the time the girl is on his bed and taking her dress off, Holden realizes he's not into this after all. Of course, she's not the world's greatest conversationalist either, so she just stomps off after trying to extort even more cash from him. To be fair, we're pretty sure her pimp is making her jack up the price.
So, why does Holden back down this time? Partly because he can't do anything, ever. But also partly because she's just a kid—"young as hell" (13.30). Because she's still a kid, she has all these little nervous habits that force Holden to see her as an individual rather than a prostitute. She's "jiggling this one foot up and down" (13.30), and says the childish "Like fun you are" instead of "Like hell you are" or "Cut the crap" (13.35); she's got a tiny little wheeny-whiny voice" (13.31); and—this really kills us—she doesn't even know enough to say "thank you" (13.31).
That's where we realize how messed up this situation is, and Holden does too. Here's this kid who's had such a messed-up, disadvantaged upbringing that she never even learned to say "thank you," being forced into prostitution by some elevator-operating jerk; and then here's Holden, rich and depressed outcast from four different prep schools about to make this poor girl's life just a little bit worse.
No wonder he can't follow through.