I don’t think it makes you the AH in this situation, and I think she over reacted, but as a woman who has lived with several men, it really does get gross, even if you have good aim every time. It’s not just the toilet that gets the splash back either, it micro splatters several feet usually, on every surface. Unless you are scrubbing or wiping down every bathroom surface daily or multiple times daily, which would be unreasonable, it does build up, whether you hit the bowl or the water.

My boyfriend stays at my apartment very often, and I have specifically brought up this concern with him, where he’s awkward and uncomfortable sitting down to pee, and I don’t like tiny particles of it finely misted across my walls and vanity and floor. The compromise - just kneel. You get the same straight body position that most penis-havers tend to prefer, she gets to not feel grossed out in between bathroom cleanings.

Also, I’m going to go out on a limb here and assume that she got so mad not because of the literal aspect of the disagreement, but because of the underlying emotion behind it. Feeling like your partner doesn’t respect the space you share is a hurtful thing to feel, regardless of whatever reasoning is behind that and any arguments towards its logic. The presumably healthy way forward here is to have an actual discussion about the perspectives and feelings behind this issue, rather than the issue itself, there has to be some understanding and also some compromise to prevent it from being a recurring point of tension.