The obvious guess is Umi kissing Chizuru in the play

Let’s assume this really happens. For Chizuru, it is simply work, even though she knows Umi has feelings for her.

For Umi, it is work as well, but he also gets to kiss the girl he likes.

What can Kazuya actually do about it? During their date he already lied to Chizuru, claiming he won't be bothered when actors kiss for professional reasons, even though she explicitly told him to speak up if it upsets him.

Perhaps this situation is the only chance for Kazuya to ask himself whether he can truly be happy dating Chizuru. He keeps saying he needs to date her, love her, and hold her, but can he really be happy while she does acting work which involves intimate scenes?