You're the Fiancé!

Innocent, optimistic Mei Sahara goes home one day to find out that she’s engaged to be married to a man she’s never met as soon as she turns twenty – a few months from now! Good-looking, well-educated company president Hijiri Kamijo looks every bit the perfect prince straight out of a fairytale, but can Mei deal with him as he attempts to take all of her firsts?(Source: Manga Planet)