I have catched up to the raws (they are scanslated in my language). And if I understand everything so far right,
It seems Yuuna also knows about Sena's feeling. She also said she loves her when they spent a night together. The problem is, it's unclear if their loves are the same. Maybe it's the same, but since she knows it's forbidden, and in order to be together, she wants to make her own family with Sena in it, she has to marry. And so hearing from Sena that she will feel lonely after Yuuna's marriage, she knows her thought is already in the wrong way. (Just my guess, her train of thought is unclear as of now. This also makes me wonder something. Is Yuuna bisexual? Does she also love her fiance? Or is he just a nice guy that she has no problem marry with? If it's the case then I have no word lol). Then come the latest chapter when we have a meeting between Yuuna and Kiku. And it ended with the start of a past, where it seems it was the start of the time they were dating. But it rubbed me in a wrong way cause it feels like they didn't even love each other. Iirc, Kiku fell in love with Sena shortly after their breakup. And like I said, Yunna just wants her own family with Sena in it, so when she noticed Sena's distance, she knew something wasn't right. Maybe she thought Sena wanted a "normal" family? That's why shortly after they were back to being best friends. And here is where I really don't like Yuuna. It feels like she just wants to do things that will be convenient for her.