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If you are troll, pic related. If not...
To the best of my knowledge (criminal justice major), there isn't any way you cast a light on this as self defense.
Self defense ended when you had him bent over on the ground. Self defense is, literally, in defense of the self.
You transitioned to the role of attacker once "something primal" took over you.
Maybe you have some sort of fucked up past or sexual molestation you can use to say that you were mentally unstable our something, but I seriously doubt that's going to justify trying to shove your dick In someones ass.
Specifically, if I was reading what you wrote as testimony, you seem to have clear motive and an intent to do something of a sexual nature when you go from CHOOSING to piss on him to DECIDING to piss INSIDE him. That being the case, i'd call any defense of your psyche into question, based on what you said there.
Tldr: you're fucked op, pun intended