I do think we're kind of hardwired differently. As an example, it's a fact that men and women typically look for different things in partners, and that same kind of psychological difference can be expanded to a lot of situations such as this one. Now, let me preface what I'm about to say with the (disappointingly necessary) caveat that I'm talking likelihoods, not absolutes:
I think that if teenage boys got to a point psychologically where they could do this to an 11-year-old girl, we're talking about mental factors that line up very closely with that of a rapist. That could certainly be true of girls as well, but I think generic quasi-sociopathic cruelty is more likely. The perception that females are weaker, or less acceptable to victimize, is a very real thing to deal with when we're talking about the mindset of the attacker. Perceptions are the reality that matters when trying to get into someone's head. Most people's sense of right and wrong are shaped by traditional societal values, and these might as well be objective truths when it comes to how most people judge right and wrong for themselves. The boys would be disregarding a stigma against victimizing women that is very similar to the stigma both genders have for victimizing children.