Sunoo could be straight I guess but idk it definitely goes beyond him being into fashion and having feminine mannerisms. He plays to the audience who thinks he's gay and even since the survival show people thought he was. He gets bashful around guys (he pretty blatantly flirted with Sunghoon on I-Land) and the way he interacts with women is very uh.. idk non descript?
If I met someone like Sunoo is person I would absolutely just assume he was gay. Maybe that would be the wrong assumption, but I think its fine to go with your gut with these sorts of things. I think the people vehemently claiming Sunoo is straight on the whole have a sort of agenda or certain baggage for why they claim that. Unless Sunoo pulls a Daehwi I think its fine to make the assumption that I'm sure he knows everyone makes.