He might meet some interesting new friends. In fanfiction, the gay club could be a place full of unique characters. Harry could engage in deep conversations about various topics like art, music or the LGBT+ community's experiences.
He could meet other fans of the same fictional universe. In some fanfictions, gay clubs are places where fans gather to discuss their favorite characters and storylines. So Harry might run into people who are also fans of his world.
He could be curious. Since it's a new environment for him, he'd likely be interested in observing the unique fashion styles and social behaviors that are common in gay clubs. He might also be curious about the relationships he sees around him.
In the fanfiction where Ash goes to the Pokemon Battle Club School, it would be an amazing place. There could be different classes for different types of Pokemon battles, like one for water - type battles and another for fire - type. Ash would be so into it, trying to soak up all the knowledge. He might even get the chance to teach some of his own battle experiences to the other students. And of course, there would be epic battles between students, with Ash right in the middle of the action.
Often, a straight guy will have a mix of reactions. He could be initially shocked by some of the more overt displays of same - sex affection. However, as he spends more time there, he may start to relax and see the club as just another social place where people are having fun. He might even find himself having a great time and making new friends, regardless of sexual orientation.
Well, Harry could be really confused at first. Ouran is a very different world from Hogwarts. He might be shocked by the opulence of Ouran Academy and the strange club system there. Maybe he'd try to find a way to fit in while still being true to his wizard self.
There's a chance he'll run into Ron. Maybe Ron is looking for some new wizarding sweets at Honeydukes, and they end up sharing some of the treats while they continue their shopping spree. And it's possible that they'll see Neville there too, who is searching for some rare plant seeds for his Herbology studies.