Holden Caullfield does not want Stradlater going out on a date with Jane, because in Holden's mind Stradlater is a total phony. Holden perceives that Stradlater is only interested in sex, not Jane personally. Stradlater does not even know her name! Holden does, though, and cares about Jane as a friend; he guesses that Jane has possibly been sexually abused by her stepfather, and worries that Stradlater's smooth-talking ways might be too much for her to handle. Holden also wants to rescue her from Stradlater's evil clutches, because of what Stradlater is-- handsome, popular, well-built--everything Holden is not. Holden would like a relationship with Jane for himself but is too socially awkward to achieve it.
Holden talks about Jane Gallagher and thinks about her throughout the entire novel. He is obviously in love with this nice, sensitive girl, but he is too shy to try to win her affection--or else he has too much of an inferiority complex to believe she would be interested in him as a steady boyfriend. Holden is "nervous" about Stradlater going out with Jane because he knows Stradlater all too well. He is quite sure that Stradlater will do his best to seduce Jane, as he does with every girl he takes out on a date. Holden uses the term "nervous," but actually he feels jealous, possessive, and protective. He respects Jane too much to think of trying to seduce her himself, and he hates to think of an unscrupulous phony as Stradlater taking advantage of the girl he loves. Characteristically, Holden does not fully understand his feelings and can only think of the word "nervous" to cover a number of unpleasant and conflicting mixed emotions.