Otona no Koi wo Oshiete Kudasai

Iroha, a new office lady at a trading company, has never been in love, but she is about to enter into an arranged marriage. However, she still dreamed of having her first experience with someone she loves. So she boldly confesses her love to Kashiwagi, the most popular man at the company! They decide on a "limited-time lover's contract" and start slowly. Kashiwagi leads the nervous Iroha, who is nervous and stiff at first, gently through a first date. Although they are only suppose to be temporary lovers, Iroha can't stop herself from falling for him...!

(Source: Berry's Bunko, translated)