@Kuraiaku To be fair, he doesn't speak of it as "if I win, Seri'll become my gf". Although someone could actually intend it that way, basically it's just a confession. It doesn't oblige Seri to accept (although she will, she's been shown to have some interest in the MC). Of course, during this fight the MC is speaking as if Seri's not even there, but he's been hit on the head enough times already to make him hardly aware of anything except his opponent.
I definitely see this scenario as related to a harmful trope, but it knows its borders.