This will be spoiler smattered so prepare yourself.

I believe Kou knew his feelings first even if he wasn't prepared to act on them. The reveal gives us a clearer understanding of Futaba's evolving emotions, but Kou never had that 'light bulb' moment. He liked her in middle school, was aware of her even when she didn't realize they attended the same high school and was first to exhibit jealousy.

When did he not like Futaba? He only denied his feelings after she gave a negative response at orientation. He's been very kind to her, against his inclination not to get emotionally involved.

Why do people think Futaba fell first?

Well, I think it's pretty hard to tell, who was actually falling first in this case, but the reason people tend to think it was Futaba could be the point of view. Because ...

Since the main focus lies with her, the reader gets to know right in the beginning many different details about Kou. These could be things, that lead step by step to the conclusion, that she started to develop feelings as the first one. For example, as she realizes the (from the other guys) slightly different way he behaves, and - more obvious - The even smaller details, like his scent, which may in comparison to each other be seen as a subtle way to express the characters interest in getting somewhat closer (not only in a physical way).

Although, it's just a guess. ^^"

"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))