pretty sure he's not straight, but Bi. I think I can remember a scene where he flirted back with me as a dude when I was flirting with him. also I'm nearly certain if you say you had "friends" during a certain event when he asks if anyone is waiting for us to come back he responds that he has had a couple as well.
and something kinda funny is I'm pretty sure Hyao hasn't made a single character that's purely straight from what I can remember lol. maybe some characters claimed they were straight but they all turned out Bi when we appeared to seduce the hell out of them lmao.