
No I think it was more the shock of her not being who he thought she was (he was told that she was just using him as she had done with other guys and remember, the third/fourth time they had spoken to each other they had s*x which was pretty sudden tbh) because that was the first time he’d ever heard anything about her from another person, esp about intimate stuff as she said “This is my first time holding hands with a person” earlier on. He doubted her for a second but then realised that he shouldn’t assume anything without talking directly to the person.

Well, she heard about him being with other women and said that’s fine and even after she saw him kissing the senpai. She forgave him. She stood by him, he should not have even doubted her. Besides, he did thought that she might be lewd because she slept with other men and didn't told him. We are so used to the thought that is all right for men to sleep around, but is wrong for women to do the same. Because of that I think he does not deserve her.
This might be an unpopular opinion, but Shinohara does not deserve FL. He slept around and judged her? The balls on him.