MC isn't exactly a paragon of dealing with confrontation. He kicks out the Fuckboi and wants to make sure that Fuckboi doesn't come back. He knows that if he starts throwing fists, he'll be the one getting in trouble (not to mention, Fuckboi is probably better in a fight, let's be honest. MC probably hasn't seen a gym in years.)
So MC engages Fuckboi.
Fuckboi is pissed. This guy just cost him his free pussy and now Fuckboi wants to hurt MC. Fuckboi knows that if *he* starts throwing fists, he'll be the one getting trouble (not to mention, MC is hyped up on adrenaline and clearly a little nuts.) So he uses what he's good at: Emotional Manipulation. So he starts spouting whatever bullshit he can to get under MC's skin. Clearly MC already hates him so the old "we're not so different" line will probably work. After all Fuckboi doesn't see himself as a rapist. All the girls he's fucked agreed to it. And that cliche does hurt MC, 'cause MC's got an overdeveloped sense of justice and as soon as he might be in anyway be in the wrong, he shuts down. Then Fuckboi spouts some more shit to make him feel better about himself and to make MC feel worse. Manipulative shit is a manipulative shit and, just like real life in a lot of cases, he gets away with it.