Why did Holden get mad at Luce when Luce called his old girlfriend the "Whore of New Hampshire"?

This is another example of how Holden sees things differently from most boys of his age. Luce clearly has contempt for his ex-girlfriend for letting him sleep with her. In his eyes, that's what makes her a "whore." But Holden has a completely different take. He thinks that the young lady was actually "decent and nice enough" to let Luce sleep with her.
Holden may be immature in most respects, but in this one at least he's way ahead of his peers. He realizes that if someone's prepared to be intimate—either on a physical or an emotional level—then they deserve respect. As Luce's comments about his ex were clearly anything but respectful, Holden finds them deeply offensive, and so he gets mad.