Words of Devotion (2000)

Blurb:
After graduating from high school, Tachibana and Otani have finally confessed their love for each and are not living together. However, their life as "newlyweds" is rudely interrupted when they run into an old, high school classmate, Yuki. Otani has always been suspicious of the close friendship between Yuki and Tachibana; and honestly, his suspicions may not be that far off...Are the feelings between Yuki and Tachibana simply an ordinary crush, or could they be more?

Source: www.anisearch.com/manga/3702