That's why you shouldn't let your rival into the harem
You just have to wonder just what is the reader supposed to believe Sara expected from her actions. Same goes for MC-kun: one minute, he's thinking of his relationship with Sara like it's fake, despite all his actions indicating he's taking it seriously; next minute, he acts as committedly as he should regarding Sara; the minute after that, he's doing things that suggest he's not in a relationship at all.
That sort of thing is why it is said that friendships with the opposite sex are unnecessary and even harmful once you're in a relationship, far more so when married: it allows for "misunderstandings" like this. But how much can you call it a "misunderstanding" when you know the androgynous washboard likes you, know she doesn't care about stabbing the girl (you refuse to admit to yourself that you love) in the chest from the front, and still accompany her on what anyone with two neurons to fire impulses between would call a date?
This author has sacrificed everything he had going in this story for a flat chest and leaning hard into donkan for its sake.