Just realised the red bean rice was referencing flicking the bean
I have to put doubt on that. That's a connotation more commonly made in Western spheres perhaps, but red rice is traditionally associated with wedding ceremonies and the "first night" that follows after in a Japanese context. Some more obscure references also claim that the pure white rice turning red is an analogy to losing female virginity.
In short, Seiko is alluding to the fact that she knows Shamiko and Momo had sex and assumes they are a couple now that will get married for sure. Just as the canon will no doubt confirm eventually (assuming Ito-sensei will ever become healthy enough to finish it).
