She's not a pervert. She's trying not to think about it. Mostly failing, but still.

"Yeah, that's exactly my point. We might be here for months, even. We'd have to give in eventually, right? I mean. We're teenagers." She motions vaguely to them both, then takes another sip of her water. Wow, it isn't very distracting.

And with that thought, Umi has probably reached a critical point in everybody's life: the moment they decide whether or not they find horrible scars attractive.