I think pissing in the sink consistently is pretty gross. Unless you’re cleaning/disinfecting it thoroughly after and let’s be honest, if someone’s too lazy to walk to a toilet to piss; they ain’t doing that. It’s likely the idea of someone being a “sink pisser” is the bigger thing here, so finding out that your partner is one (a slob) is probably what led to the reaction.

Bringing up what you think about a partners reaction to something can happen later. Saying “it isn’t hurting anyone” is marginalising her feelings instead of understanding what lead to it and is unhealthy/unhelpful imo. It’s a cop out. I could scoot my bare arse across the carpet in lieu of toilet paper and say it isn’t hurting anyone (ignoring carpet burn). Wouldn’t make it not gross as fuck. That’s not to say she’s chosen the most effective method for voicing her issue but that’s an aside.

not sure what even to say in that moment

I’d suggest he start with apologising for pissing somewhere that is not the designated piss zone, aka a toilet and go from there.