I can't tell if Chiaki is actually sexually aroused by boobs, or is it just the usual platonic boob fetish, anime and manga girls tend to suffer from.
If it was a boy there would be no way to hide the arousal. "Is that a gun in your pocket, or are you just glad to fondle my boobs?"
If it's a girl, there's no easy way of telling -- so she can claim her feelings are purely platonic aesthetic appreciation. But, in reality, that's not a thing. There's always an erotic component involved when you grope and fondle another person's body, especially if you're a teenager.
No matter how strongly Chiaki denies it, the "energy boost" she feels when groping Harumi is probably sexual in nature. And sublimating that through activities such as sports is most common and really typical. It's, like, textbook psychology. Right?
