Nate can't believe that, but what he can't be blamed for not understanding is that, while all three of these things above seem really trollish/sexist/3OH!3, they're all completely off-base. Blair isn't ranking on Bree because she's a hater, or because she still has feelings about Nate, or because women are bitches: She's ranking on Bree because Bree makes her feel prickly. The issue is that she's using Mean Girl tactics -- the hostility, the rudeness, the Nate territorialism -- in service of non-Mean Girl ends. This isn't directly about breaking Nate and Bree up, it's about getting Bree the fuck out of there, for the good of them all. Acting as the immune system for the group, just like always; just like she and Chuck were doing at Sotheby's, before Carter and Bree even looked connected. But because the tools are the same tools she'd use anyway, nobody can figure out what she's doing, including Blair. But she's just using the tools she has.
"When bacteria produce poisons (toxins) in the system, the cells affected are stimulated to secrete an antitoxin which counteracts the influence of the toxin."
Chuck walks into the kitchen as Bree's getting off the phone, having explained to some Texan that Carter is in the vicinity, and won't be going anywhere because he's in love. Chuck tells her that "the worm" is dating his stepsister, and tells Bree to level with him about the Carter threat: "Any enemy of Carter's is a friend of mine." She can't talk about it, but tells Chuck to make sure Serena watches her back -- and to let her know if Carter shows up.
He's just acting as the immune system for the group, as always. He's hated Carter since before the show started, because Carter gets to be the same kind of person Chuck is without ever seeming to risk his reputation, and because Serena never questioned him or his redemption, and now Blair's okay with him too? Because from Chuck's perspective, Carter is the Georgina of boys. Trace it: Nate loved him and Chuck hated him, just like Blair was always jealous of Savannah and Svetlana. Season One, Georgie came back and essentially kidnapped Serena, or her life, just like Carter did Nate (the baseball thing). Carter took Blair away when she was going crazy last year, just like Georgina did with Dan before OMJC.
Blair and Chuck are the only two people who are allowed to transcend their histories. Every time either of them shows up, he knows, it's only a matter of time until they show their true colors, and now that he and Blair are a team, they have to close ranks on this. So to see Blair going after somebody other than the enemy -- harboring the enemy in fact, which he doesn't know yet -- is not only a betrayal, but a dangerous loss of focus. Especially when Nate's so happy.