In Chapter Nineteen, Holden admits that the main reason he called his childhood friend Carl Luce, besides wanting some company, is that old Luce was always good for a sex talk. Apparently, that's all he did at the Whooton school as a student advisor. It looks to us like Carl used sex as a way to make friends and get attention. As Holden says, he never wanted the boys to hang around and talk if he wasn't there – he had to be the big shot running the show.
Unfortunately for Holden, Carl isn't so much into the open sex talks anymore. He's far too mature for that now. Luce is all "I'm SO sophisticated and worldly I can't even talk to you right now." Ugh. The funny part is that Carl obviously wants Holden to know about his sex life. Carl lets it drop that he's dating a much older woman, that she's Chinese, and that he's into the Eastern philosophy of sex. Of course, he ends all these little slip-ups with a "If you must know."
What's interesting is how Holden acts around Luce. For the first time, we see Holden as a badgering kid, instead of as a pensive, compassionate guy. Look at the line, "Wuddaya mean 'Philosophy'? Ya mean sex and all? You mean it's better in China?" This isn't the Holden we know and love. But in this scene, we also get to see how truly lonely he is. As annoying as Carl is, Holden begs him to stay for another drink ("Please. I'm lonesome as hell"). Perhaps, then, it's no surprise that this little episode directly precedes Holden's drunken wandering in the freezing cold park.
There's also the question of whether or not Carl is gay. Holden seems to suspect he is, or at least to have suspected at some point in the past. Luce, he says, was always talking about other people being gay, even those who were quite blatantly not. He also engaged in what Holden called "flitty" activities around the dorm. It could be that Luce is gay. Or, Luce isn't gay, in which case Holden needs to stop suspecting this of men he knows.