OK. The Occidental College case happened in California. The girl was 17, and in California, the age of consent is 18, so in the state where this happened, it was statutory rape. That is a particular aspect of this case that exists because it happened where it happened which would not exist had it happened in, say, Indiana, where the age of consent is 16. It is important to bear this in mind.
Second, colleges can do what they want. They are free to have codes of conduct that are over and above the law. For example, the dorms at Indiana University are dry, and that includes the graduate student dorms where everyone is over 21. IU is free to expel a student for drinking in his dorm room, even if he is 23 and has legally obtained alcohol that he would be free to drink in some other location.
Schools are, for that matter, free to have “No sex whatsoever” policies, and enforce them, and for all I know, there are Christian colleges that do this. School are free to have dress codes, and expel students who refuse to conform to them. A school can expel a student for doing things that “defame” the school, even if no criminal charges are filed, and it’s happened. I know of an entire fraternity that was expelled for overtly racist acts, although in the case I’m thinking of, the students were given an option to reapply after two years, so it was something in between an expulsion and a suspension, because they could come back, but they couldn’t automatically come back, just reapply. The fraternity itself was permanently barred, though. Not a single thing that any student did was illegal, just grossly inappropriate.
So in short, the male student was not charged with a crime, and probably could have been, meaning he didn’t “have the book thrown at him,” but he was expelled, and that is up to the school, which is free to act over and above the law.