
*SPOILERS*
It's left implicit that he does have some type of sexual experience and he did it with a man. Even tho it's not clear what exactly.
The virgin part is said in the side stories when there is a task where the person doing it needs to be virgin, Seo gets nervous and ask what virgin means, when it's said that means penetration he is clear to pass the task.
About liking men, it's a short conversation with Jihan before entering the great dungeon 2, I don't remember were exactly.

I think he just- you know, kinda jerkin' off together. There's no penetratif action. And seo got confused to think if he's a virgin or not. This is what i get in a korean site that talking about this novel. And yes, a lot of people also think that seo really did it with someone in the great dungeon.
Was his friend his first love or something? Were they together?