TLDR finally MC goes out with his crush but the other guy sends him a video of him doing his GF, turns out they had a sex based relationship that it's supposed to end that day as it is implied he is blackmailing her for sex now that she is going out with MC, after the video ends he goes to her house and finds them both still doing it, by the end MC decided to keep shut about the photos and video as he tells his gf he loves and she replies she love him too unaware that MC knows now leaving it open if she will keep doing it with the other guy or she stuck to her words (doubt).
It's too sudden because it doesn't show how her relationship started with a guy she says she hates to end up questioning why she hated him because sex was better than her BF who just lost his virginity to her.
A story about a serious older sister and an innocent younger childhood friend. Rin, a year older than Yuuta, goes to high school first and as they spend more and more time apart, Yuuta realizes his feelings. A year passes and Yuuta also goes to high school. He finds Rin and confesses his feelings. The two deepen their love, trying to make up for the time they lost. However, the more he gets to know her, the more he begins to feel uneasy about a side of her he didn't know... One night, Rin is cheating on him with a strong and manly guy who is different from Yuuta. He is made to understand the differences between them...