In Nothing, sex unfortunately equals rape. Some might argue that Sofie consents, but it's clear that what goes down in the warehouse is an act of sexual violence that raises some hairy, scary questions about what exactly consent means in the first place: if you go along with it because you know your peers expect it of you, are you really doing it of your own free will? Certainly by the time Sofie gets to the sawmill and it's her against four boys, she has no choice. However, the next day at school, she acts as though she has an important secret, and she refuses to give Agnes details. Agnes even compares her to a martyred saint. There are no easy answers about sex in Nothing, just a lot of difficult, uncomfortable questions.
Huge Hans had sex too, but we don't see him reacting like Sofie. Either he wasn't a virgin, he didn't have a preexisting mental illness, or it's different for boys in this book.
Sofie refuses to talk to Agnes about her experience at the sawmill. She could be keeping her secret because she's discovered true meaning, or she could be dealing with shock and trauma and is unable to talk about what happened.