The kiss is an important, but not the only reason why i would think that is more Sekai as Kaori's doing/feelings. I mean, i cant recal exactly what it was, but i am sure she got some
transplant after the accident. And while i am not sure if it was ever directly implied in the game, i did read that others also believe that it was from Sekai. So Sekai is a part of her for a long time.
And nothing of her personality ever did show her as a particularly sexual person. So she having as one of the first reactions one of been physically attraction to a man she didnt knew, doesnt strike you as strange? Or maybe even out of character? Because it does for me.
Technically you are correct and they are all adaults. Only that all except of Kaori know what they are doing. They are aware of what sex means. They made the conscious decision to engage with him, with all the consequences that would include. But Kaori is imo just technical an adult. The way she does see the world and reacts to it are quite often child like.
I never called her a literal child. But mentally she is not an adult who is fully there and in full possession of mental decision-making abilities. And that is my problem. The fact that there is another person inside of her that might take over just doesnt help. Reminds me a bit of a story of a women with another personality that sometimes takes over. And then gave control back after bringing her into a situation where... well lets just say for the men involved it wasnt rape because "she" gave full consent before putting the gag in "herself". It wasnt a story with a happy ending. At least not for the original personality.
Oh, you sweet summer child. I have trouble believing all on this page would agree. But my problem is, that currently as it is, i just cant see any scenario where he would not take advantage of her in some form. I know it will happen regardles of what wish for. But, doesnt mean i have to be happy with it.
