@DeusVult The whole point of a previous arc was that love doesn't need to be anything grand, and they don't need to go out of their way to make it grand, it can just be simple and cute, to the point, and that's what Kaguya wants, normal. A life full of frivolous or even grandiose things is not what she wants, since she lives in extraordinary circumstances already, so this event feels like par for the course.
Plus, it's only natural that they let themselves go once they started cuddling and kissing and just went for it despite the heart to heart.