I'm playing three versions, Dom top, True versatile and Sub bottom... I hope you didn't try to sub or bottom shame on a gay game, but back to your question, your Player Character DID NOT defeated the Warlord, you just weakened his physical form (since he can reform himself) for long enough to take his god-like nature (Anger) from him before he has the chance to reform his weakened body... hes nature is dominant by nature, no matter if you PC is top or bottom, Dom or sub, Arihama does what he wants...
I said you had sub playstyle because there are different options and different ways the story plays out based on role and side quests (like the Akonian ritual). I'll believe you that you don't win when playing as sub or skipping/making wrong choices in side quests/main story.
In my playthrough as dom top with all side quests and main story done with correct choices, I have the option to destroy Arihama's mind after destroying his body, which would completely destroy him, he can't reform from that.
Here's a screenshot in case you don't believe me:
1 year ago(+1)(-1)
This is after you started sucking/extracting Arihama's nature... It's after the weakening fight, you win by robbing him, not by fighting...
1 year ago(-2)
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.
1 year ago(+3)(-1)
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.