On the side of the rapists. Via an interview she gave to Rolling Stone:

We watch the news for a while, and the infamous Steubenville rape case flashes on the TV – two high school football players raped a drunk 16-year-old, while other students watched and texted details of the crime. Serena just shakes her head. “Do you think it was fair, what they got? They did something stupid, but I don’t know. I’m not blaming the girl, but if you’re a 16-year-old and you’re drunk like that, your parents should teach you: Don’t take drinks from other people. She’s 16, why was she that drunk where she doesn’t remember? It could have been much worse. She’s lucky. Obviously, I don’t know, maybe she wasn’t a virgin, but she shouldn’t have put herself in that position, unless they slipped her something, then that’s different.”

They “did something stupid?”

No. Filming yourself trying to skateboard down stairs is “stupid.” Filming yourself sexually-assaulting a drunk girl is a crime. Serena should be ashamed of herself.