It's most likely he is being sarcastic. It is like some kind of ridicule reaction, of all the time they spend together now Jinha (MC's name is Jinha btw. The character you said is Won) questioning Won like he was some kind of whore who happily sleep between brothers.
it'll get explained in later chapters but (spoiler)
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he purposefully said that knowing it would hurt mc, bc mc would think of him more/be more obsessed with him this way. he says all these knowing how it would drive mc mad for him.
Pls just ignore my comment. I just comeback from scrolling further and see the spoilers. It's a fucked up story man (￣∇￣")
I really don't get why Jinha lied to MC in the previous chapter (ch.29). Why make him suffer? What that does he gain from it?