Because their conversation before was about the definition of what a friend is, how it's not someone you sleep with, but the ML kept insisting he's just friends with MC and not fuck buddies, then in the same conversation he asked the glasses guy to be friends, so glasses guy inferred that meant ML was asking him to be fuck buddies too. The chapter cuts off before we see what happens so whether they actually messed around or not we're waiting to see... As for whether they're just pretending as ML told MC they are, we don't even know what ML's definition of "pretending" is - like are they pretending to be friends? Pretending to be fuck buddies?

okay so, i don’t usually comment but i feel like i need to i’m beyond confused, did the black haired guy (i’m
not good with names) sleep with the guy with glasses or no? or is he lying?