I'm guessing from the hints in the previous chapter (the TV report about a teacher having sex with his student, and him saying "I'm the worst") that Yuu is feeling absolutely disgusted by himself for being attracted to Marika.
I mean, she is 6 years younger than him. It's not that much of a gap when you're both old, but he started "acting cold toward her" (a.k.a. he started having feelings toward her) when he was in high-school. If he was 18 by then, then it means she was 12. He was basically a pedophile.
She isn't seeing it from his perspective. In her view, he is her childhood crush. But in his view he is doing something forbidden. Even though they are both adults now, it doesn't change the fact that she was a kid and he was an adult when he fell in love with her.