Kou never genuinely asked him out; he only asked to stay in contact. Chiaki probably rejected that idea due to not knowing if Kou was gay or not + his ex was blackmailing him... Or so I've gathered.
I think he rejected Kou when he offered his contact info because Masaki didn’t want him to be in contact with anyone but him. Maybe he was just saving himself have having to see it be deleted and being isolated once again? Maybe he was protecting him too.
I really wanted to know why he rejected kou Chan.... I’m guessing something to do with masaki??? His work??. There’s wasn’t a proper explanation