The further into this story we get, I get more convinced that the author is portraying Yuu as either Demisexual or asexual. She truly doesn’t get what it is like to be sexually attracted to ANYONE. However, I get the feeling she is opening up to a romantic attraction to Nanami sempai. Once the romantic attachment is stronger, she could start to have sexual desire for her as well. (At least I hope that is how the story is going.)
Reading some of the comments here, it seems other people have thought that too... I am really impressed if the manaka really is intending to address either asexual or demisexual (or demiromantic) issues.
My personal take, which means precisely nothing, is that she is demisexual. I think she has already developed romantic feelings for Nanami but is confused because she doesn't feel any sexual desire for her, despite wanting to support her, be by her side, and spend more time with her. I really look forward to the chapter where she realizes her true feelings! So far the mangaka is doing an excellent job with this manga, in terms of art, story, characterization, and pacing. I love it!