I'm pretty sure Ruriko slapped her as a kneejerk reaction when all of her feelings reached a boiling point and subsequently exploded. Not to normalize or justify physical abuse, but this is certainly not what I would consider it as. For one thing, Kurumi is the drunk one and she suddenly grabbed Ruriko to kiss her on the cheek, something that is seemingly out of character for her. Sure, Ruriko could've simply shoved her away, but I think her slap was meant to convey the same thing, and it's more dramatic anyway.
I know there are a lot of people rooting for their development and view this as a frustrating step backwards. Honestly, there were probably a dozen different ways they could've revealed their feelings for each other, but hey, guess we're going the drama route, which I think is fine. The two of them aren't perfect, and pretty much every single chapter shown from Ruriko's POV is about her feelings towards Kurumi and her insecurities surrounding them. She's spent their entire marriage anticipating the day Kurumi would inevitably leave her and tried to convince herself she was okay with this, but in recent chapters (including this one), she's become more greedy in hoping that she could be special to Kurumi and not just another one of the many things Kurumi loves. Obviously, this is not healthy for Ruriko, who also has no idea that Kurumi has come to reciprocate her feelings little by little.
So this chapter's slap, while it could've been handled much better, kind of makes sense to me. Not only did the kiss on Ruriko's cheek catch her off guard, but Kurumi was totally wasted. There's the possibility that Ruriko was frustrated that her feelings were being treated so lightly (not Kurumi's fault, since she isn't supposed to be aware of Ruriko's romantic feelings for her), that Kurumi could kiss her so easily while being drunk of all things without taking into account how Ruriko would feel.
Honestly, I have no idea, but I'm pretty sure a lot of little things welled up in Ruriko in that one moment and it literally exploded all at once. That girl has been shouldering a ton of anxiety ever since they got married, so I'm not even surprised. But seeing how these two have been able to sit down and talk things out together (the trash incident, the hospitalization, etc.), I'm sure they'll be just fine, even if it hurt a lot in the moment.