I think

This^

I thought Xuan originally wanted Jinha anyway, and he only started to tease Siwon in order to try and understand Jinha more and retain a connection.

Lol. "kind of ignorant" -> Lol for defending him that he's not stupid when he could've avoided all of this if he wasn't stupid enough to go with the Chinese dude in the first place. It's his fault for being ignorant.

But Xuan RUn manipulated Siwon to go with him making him think he was gonna tell Siwon some big secret about Jinha

Oh no, he's definitely stupid hon. Look at what you just said, "he said he'd tell him a secret about Jinha if he came with him", is that supposed to be a counter? Before that the guy kidnapped, bound and graffiti'd his best friend and also put that picture up of Jinha and him kissing. Jinha warned him to just stay away from Xuan too. Xuan appeared in the dreams and messed with them. This list goes on with smaller conflicts like sour interactions between him and Xuan as well as him seeing Jinha upset around him. There is no argument for Siwon's brain, not just that, but his character is clearly meant to be naive. Like how he's been kidnapped to an isolated island, but has only tried to escape once, how when he was trying to escape he got worried about Xuan's injury, how he's just eating up whatever Xuan tells him about the stone and Jinha, etc. Siwon is trusting to the extent that you can see he's supposed to be a man-child.

I agree he's too trusting yes he's still young of course he's naive but that doesn't make him a over all stupid person, I mean do you have life completely figured out? He's been through a lot that takes a toll on someone, he didn't even understand his feelings for Jinha until recently and then that got shut down because Xuan Run put up the photos of them kissing and that made him vulnerable again. Yes I think he could learn to think things through a little more but he's a simple human being he cannot control everything and his feelings are what they are, he is who he is and he's still learning. So think about it do you wanna get told you're a stupid person every time you make a mistake? We are humans were meant to make mistakes and learn from them so just cut him some slack he can't even do anything right now he's on the middle of a Island stuck with a very manipulative person who will do anything to get what he wants so I'm not entirely sure what everyone wants from him right now, he's a target and that's not just gonna go away once he has everything figured out Xuan Run won't stop just because Siwon tells him too. That's really all I'm saying I guess most people wouldn't really get it though because not everyone's been a victim of stalking which is basically what Xuan Run is doing to Siwon I guess people are just really naive to this kind of topic.

and btw I said he was "Kind of ignorant to understanding his own feelings." which is completely different from just overall ignorant, I think he's just young and naive but aren't we all, lol.

But it's not "every time he makes a mistake", it's after a series of successive mistakes. It's pathological. THAT is what makes him stupid. I know everyone makes mistakes, but i don't feel like that excuse can validly be applied to Siwon, tbh. At what point does it go from "occasionally making mistakes" to being full-on stupid? Maybe stupid is too harsh, i'll say naive from now on.

but he is still young and experiencing something he's never felt before.

He's in college, though i'll give you that he wasn't ready to be outed like that. I don't think it's being gay that he has trouble with at this point, i think he got over that when he talked to his future self and saw Jinha almost die, what i think the issue is is that he got outed to everyone and suddenly had to face the prospect that society might look at him weird if he's open about his sexuality. I think THAT'S why Siwon was so hesitant with Jinha after the photo and why they broke up.