It's weird because at the start it only looks that they're dating because she's digging him for that council power, and now you do this, like how dare you, also I love them now, thanks author
Why can't all romance manga be this smooth with confessions? No misunderstandings. No lying. No third wheel love triangles. No shyness that causes you to not confess for 90+ chapters. No "Does she like me? Do I like her? Does he like me? Do I like him?" Just simple clear communication of feelings: "I like you." "Guess what? I like you too!"
Reimi was probably used to guys approaching her and ignoring her best friend Sakura, so when Sengoku comes along and treats both of them well that pretty much sealed the deal for Reimi.