Am I confused? Probably. This situation has nothing to do with a gay-perceived straight relationship. These girls are married. They present to the entire world as a lesbian couple. Even if they don’t think of themselves as anything but two people, they have explicitly put themselves on record as thinking that a same-sex relationship is fine and normal.
So why the slap? It seems totally out of character for Ruriko. And now I get confused, because I am unable to differentiate what I think Ruriko’s reaction should be and what mine would be. If I were in this situation, and was suddenly kissed by the woman I love, I would certainly not hit her. More likely would be that I would collapse in maelstrom of tears of happiness-confusion-fear of it not being real.
The conclusion. It’s a painful and unpleasant plot device.