Yuu is demisexual. She thinks of love based on what she read or heard from books and songs so basically, when those things defy her expectations of love, she acted like "No, I wont be able to fall inlove coz I cant" and thats her character until the cultural festival. If you will read her actions and not just her dialogue, you will realize she started LIKING Touko but she NEVER REALIZES IT FOR A WHILE. As to where?
1. (Level 1) During the campaign rally, they acted as a couple, the "past revelation thing". Surely, if you are in Yuu's shoes, will you just allow someone who openly like you to get indulged by you like that? Caressing her hair,letting that someone bury their face on your shoulder? Aint those a couple thing?
2. (Level 2) When Maki found out their secret, the first thing she imagined was Touko being in a big deep trouble and she doesnt want that to happen to her. Now, again, if youre that person, will you not try to protect yourself first if someone saw you kissing someone you dont even actually like(insert question mark here. will you just let someone you dont like kiss you and you allow it coz you enjoy it btw?) and youre not even dating? People who are not having special feelings for other people will just say "hey,its not what it seems. its just for fun" But Yuu was terrified with the idea that Nanami might be in big trouble once their secret will be out there. Maki chalks it up to her being defensive about her girlfriend. Yuu "knows that isn’t the case"(actually, she doesnt know,do not be blinded by her monologues).
Her immediate reaction to want to protect someone who loves her shows us that Yuu does care about her. If her actions mirror those of someone in love, are the 'special' feelings really necessary to say she's not yet inlove?
3. [LEVEL 3] The river scene. When Touko said she would rather die than to give up her sister's character, thats the nail to the coffin for Yuu. Yuu underestimating Touko and got ahead of herself knowing Touko's inlove with her but she didnt see that coming. When Touko said things will happen even without her, read her decision and action. If you dont hold special feelings to someone and they acted that way to you, wouldnt you say "Fine. Suit yourself. I dont like you anyway so why would I be bothered by it?", aint that right?
But Yuu?? Her reaction and her decision to "continue" to stay by Touko's side tell us "she already chosen Touko to be the one she wanted to fall in love with". She already like Touko. At this point, she already developed the feelings but she herself is not aware of it coz she's a demisexual. You will not simply allow someone you dont like to make you do things they like,kiss you and indulge them coz you just want to 'understand' that special feeling. Choosing someone and hoping you will fall inlove with that someone already mean you like that person coz you choose to stay beside that person. You will not just randomly choose someone and say "Hey, I want to experience love, I choose you".
There is that 'feeling' atleast eventhough you yourself is not aware of it.