Percy Jackson is the player of Goode High. He has purely sexual relationships with girls and dumps them before they can get too close. He's considered a heart-breaker, and he's fine with that. After all, this title is better than everyone finding out his secrets.
Annabeth Chase is a fighter. Even while dealing with personal stressors of her own, she is strong and always stands up for her friends. Her passion and pride lead to a confrontation with Percy Jackson.
During their heated argument, Percy jokingly offers a proposal. However, Annabeth always despised Percy, so she jumps at the chance to serve some payback. The proposal is simple: Percy cannot have sex for three months (other conditions applied).