This is after you started sucking/extracting Arihama's nature... It's after the weakening fight, you win by robbing him, not by fighting...
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It doesn't really matter in what way Arihama is destroyed, winning is still winning and destroyed is still destroyed.
To me Arihama is just another npc like Bradley. Bradley is all hyped up to be an undefeatable warlord with a pirate army and like 10 different war titles, but I still beat his ass and made him my bitch. The same goes for Arihama, now the only thing missing is a good sex scene to dick down Arihama and teach him his place, before sending him off to the void/completely destroying him.
Winning the fight is irrelevant to the point you are making. The sex scenes occur during the fight while Arihama is still much stronger than the player. All you have done is make him extremely horny, while he is stronger than you. It would be very out of character for him to be submissive at this stage. At the end of the fight, yes it would be in character for you to be dominate with him but you don't have the time to have sex before he gets back up.