Because the girls are Socs, Ponyboy is uncomfortable with the way Dallas speaks to them. He knows that this is typical behavior for Dallas and even admits that had the Cherry and Marcia been "greaser girls" he would have joined in. Still, the fact that they are Soc girls and they attend his school makes him uncomfortable and reluctant to assist Dally in "talking dirty" to the girls.
Ponyboy's reluctance is further evidence that Ponyboy views the Socs differently than he does himself and the people in his neighborhood. Throughout the novel, he insists that he is just as good as any Soc, but it is obvious that deep down he doesn't feel that way initially. It is possible that he is jealous of the lives the kids on the Westside live. Perhaps after being treated like a second class citizen for so long, he has begun to believe that is what he and the people in his community are. Why else would he show respect to Cherry and Marcia when, according to him, he wouldn't have minded if they were greaser girls.