The sempai was raped by an upperclassman in the astronomy room, now he's scared of being pushed down. He told the whole school that he was the one who seduced the rapist so that people would not get involved with him anymore (he was propably scared of being betrayed again if he got too close to people).
I think he's okay with the glasses guy because he was already his friend before the incident and their relationship has always been casual.
The MC is the only one who puts two and two together (regarding the rape), that's how he understands that if he "touches" his sempai he might break him (because it will revive the trauma).
Thought the same
OH
i did not get the last one xD