I reread the story. You're right, and that certainly solves the tag issue - but the whole rest of the story becomes a very different, and darker, piece. Yumi is revealed as a sly manipulator that has been stringing Yoeko along for years, since childhood really, by pretending to be an airhead. Even after having a boyfriend and having a child by him, she is still keeping a grip on Yoeko as her security blanket.
Given the title, I suppose I should've been able to figure it out on my own, but my mind was bent on casting this as another sad "unrequited love" story. So I was definitely wrong - but it also seems to put Yumi firmly within the "depraved bisexual" trope, so I'm not sure how much of a good thing that is.
I think that's an uncharitable reading of Yumi to say the least. Mainly, I don't think that Yumi has been leading Yoeko on this whole time because Yumi says "ever since that day" on page 14, implying that she only fell in love with Yoeko when Yoeko stepped up for her when Yumi's parents died and her boyfriend left her. Yumi was pregnant at the time, and her daughter is 3 in the story, so presumably Yumi has only been in love with Yoeko for about 4 years. To speculate a bit, it kind of makes sense that Yumi would not have seen Yoeko in a romantic light until then, since there is such a huge age gap (about 14 or 15 years?) between them, and since they had known each other so long.
It really boils down to at what point the thought blocks stop being in Yoeko's voice and switch over to Yumi. I think the last one in Yoeko's voice is on page 11, where she says she didn't expect Yumi to get pregnant. After that, the thoughts are all Yumi's, except for the last page where it is both of them. Yumi asking if she is being unfair is her thinking that it is wrong for her to rely on Yoeko's previous promise to protect her as a means of preventing Yoeko from leaving her, since Yumi has probably not confessed and seems to see herself in a negative light (and probably as an undesirable romantic partner) based on how she talks about herself.