Uhhh... She could have declined. She could have rejected the confession the moment he walked in. Shunin obviously wants to believe in Shibata, but maaaan is she not really trustworthy. Every time she hasn't believed in him it was a mistake, but she still gets swept up by sweet words. And she then has the audacity to get mad because he... takes what she says seriously? Shunin isn't perfect, but damn is Shibata fickle and quick to make big statements but not talk things out.