What, don't tell me you don't remember that ep? Shuji Kenji (Harimas little bro) turns up at the Tsukamotos during a storm. They take him in for the night, and Yakumo sees that he really likes her (in a platonic manner). So she decides to lay down beside him for the night coz she's a softy for little kids.
tl;dr: Yakumo is pedo for Harimas brother.