Majokko Vivian

When a witch on the legendary island of Avalon reaches the age of 14, they are accepted as an adult and allowed to marry human men. Vivian is chosen to marry a Japanese high school student named Akari Ikkyu. Vivian is delighted to find that she fits the bill, but her best friend, Araune-chan, is also married to Ikkyu!

(Source: Shueisha, translated)