"Potteto Salad"
Mmm... I see what you did there.
I love how gramps went in for the save there. Nothing like a little dose of wisdom and insight from your elders to help this young (future) couple. She's understandably disappointed and miffed about the dinner, but this is a side of Wakana that Marin will have to be able to deal with, if she really does want to be with him. Likewise, she also has her own quirks that he should prepare to handle.
A few people are reacting to Wakana like he just ignored his girlfriend, but... they aren't even dating yet. I'd be pissed too if this was after they've already started going out, but it's not. It's not like Wakana is deliberately and maliciously ignoring her, it's extremely obvious he's still tunnel-visioned on his work and he's the kind of guy who just can't help but get too absorbed into it. It could also be seen as a personal flaw of his that gives him room to improve on as a character.
Sure, this is a step back in their relationship, but it also sets up the possibility of him having to make it up to her later on.