What was the fight between Stradlater and Holden really about in The Catcher in the Rye?

Holden has issues with sex and manhood, and both are
issues when he learns that Stradlater and Jane are going on a date.  Since he can't
communicate his disapproval of the match to them or himself, he lashes out in childish
behavior, knowing full well that he will be punished the bully
Stradlater.

First, Holden does not want the adult world to
encroach on his childhood innocence.  He's in a state of maturation denial.  And he
doesn't want others to reach adulthood either, especially a childhood girlfriend like
Jane.  Stradlater clearly believes the opposite: he uses sex as a means to experience
the adult world sooner than he should.  So, when Holden learns that Jane is about to
cross the threshold from childhood to adulthood by losing her virginity to Stradlater,
he expresses hostility.

Holden also is a masochist.  He
likes to see himself be victimized by other males in an effort to protect children and
girls.  He's playing the role of James Castle here: a chilvarous defender of innocence.
 Later, Holden will get beat up by Maurice because he refused to pay Sunny.  In both
cases, Holden likes to see himself bloody: it's a overture to suicide.  He's a character
in his own sadistic gangster film in which the bullies of adulthood beat up the innocent
victims.  Again, all of this is repressive behavior in an effort to remain in
childhood.