That's one of the things that makes me mad about both the novels and the anime: There's baiting for the sake of popularity. Even in the novels, where Shuichi is more prominent and there's something resembling a "relationship" between him and Kumiko, Reina and Kumiko are way more important to each other than anyone around them. Which is why, like so many people, I was so pissed off when they pulled the "I like Taki-sensei" card to kind of justify Reina NOT being gay, and then they completely forgot that point. Correct me if I'm wrong, Thiaguinho-sama, but they never mention it again in the books (I have terrible memory so I like to make sure). It was made just to tie the lose ends so Kumiko and Reina didn't ended together.
Value dissonance dictates that we, as westerns, don't think the same way as Japanese people, where girls "train" each other for relationships by playing gay in highschool. Which is sadly why KumiRei will never happen in canon, and which is why it's so maddening what KyoAni is doing. Want to make a nice, absurdly pretty anime about a high school orchestra, with a lot of character development and a somehow believable romatic arc for the main character? Then do it properly, but don't bait us this wat for the sake of popularity. It's such a dick move