Thats a good move. actually not pulling through with the Sex that she want to be as the first and last time would make her waver more about the marriage as she didnt get everything she wanted to get that closure she needed to accept the marriage.
the only way now is they should probably confront the grandma himself. much better if both of them would talk to her grandma. knowing her Grandma, she would want Chizuru to be Happy so Chizuru and Rin should both confidently convince her about their love and future. then grandma can like pass away happily or whatever.