Yuuna basically used Kiku to "create a family" with Sena but never loved her. now she found her true love, a man, so she can be happy in an actual marriage and have kids. Sena never loved Kiku but the author is teasing it as the opposiye now. Kiku always loved both so that's why she's just going along with their BS.
Honestly, these yuri authors are probably all in a hetero relationship or just hate lesbians in general since they always seem to make the lesbians a loser lmao.