Jelani Maraj (who I had never heard of) could be imprisoned for a long time for "sexual assault". What does that concretely mean?

Due to the vagueness of the term "sexual assault" together with the dishonest law that labels sex with adolescents as "rape" even if they are willing, we cannot tell from this article what sort of acts Maraj was found to have committed. So we can't begin to judge whether those acts were wrong.

I see at least three possibilities. Perhaps those acts really constituted rape — it is a possibility. Or perhaps the two had sex willingly, but her parents freaked out and demanded prosecution. Or, intermediate between those two, perhaps he pressured her into having sex, or got her drunk.