From Midnight Scans:

Sayuki is 16 and has never had a boyfriend. Since she has no encounters in her all-girl's school, should she go to a gokon? That Sayuki made a boyfriend! Her partner is her cousin, Takumi-chan. Takumi-chan is an adult and very popular. He's probably just teasing Sayuki! But the throbbing of her heart won't stop. Unintentionally, she says something she doesn't mean?!