@Darudius
The text is more likely coming from him, but arguably it may not even be referring to the kiss, but the conversation, where he's asking her to "stop" more in the sense that when a loved one's mind is in a bad place, you might ask them to stop talking after an interjection like that. That, or he lies here to protect Koto, which seems consistent with his character, especially as earlier we see it was Kyousuke who was trying to say something and making the grunts
Given all the speech bubbles here lead to him and she appears to be rather firm and emotionless compared to him in this scene, I think it's fair to say he is the one speaking here.
(Note, "the moon is beautiful" is a double entendre in the Japanese language because it sounds and looks like "I love you," which is a subtle way of confessing)
@starlightcitypro Important distinction here, legally. Koto is not a "child," she's a minor because she is below the age of 18, which is the age of consent in the Tokyo prefecture, (She seems to fall into the 16-17 range which would be around the age of consent in the West given a first year of High School would be around the age of a Sophomore in America) Even then it is only legally in regards to consent to sex, which they have not done and Kyosuke actively has dissuaded her from in the past. Paired with the fact that the law in Tokyo makes exception if there is demonstrable love, and he's in the clear.
The most indecent thing they have done with both of their consents is her kissing him, which is not illegal. The police are just here to establish what their relationship is.