Kuzu no Honkai

Seventeen-year-old Mugi Awaya and Hanabi Yasuraoka appear to be the ideal couple. They are both pretty popular, and they seem to suit each other well. However, outsiders don't know of the secret they share. Both Mugi and Hanabi have hopeless crushes on someone else, and they are only dating each other to soothe their loneliness. Mugi is in love with Akane Minagawa, a young teacher who used to be his home tutor. Hanabi is also in love with a teacher, a young man who has been a family friend since she was little. In each other, they find a place where they can grieve for the ones they cannot have, and they share physical intimacy driven by loneliness. Will things stay like this for them forever?

(Source: MangaHelpers)

Included one-shots:
Volume 1: Nyan Nyan Prelude
Volume 2: Nyan Nyan Serenade
Volume 3: Nyan Nyan Oratorio
Volume 4: Nyan Nyan Cantata
Volume 5: Nyan Nyan Andante