There is But One Ultimate Way

There is But One Ultimate Way (たったひとつの究極的なやりかた) is a Side Story that takes place after the Alicization Arc. It was written May 2010, and was included as the 7th side story in "Sword Art Online Material Edition: The Assemblage," released 2011.

Plot
Kirito awakes in a bed in Underworld and soon realizes that he's surrounded by Alice, Asuna, Leafa, Silica, Lisbeth, Sinon and Yui. After they wake up they start talking and Kirito asks for answers about what's happening. In the conversation they reveal their uncertainty about the future, as Asuna and Lisbeth are about to graduate, and chances for meetings may become rare.

As such, they want to spend the last of their holidays together. All the girls confessed their love for Kirito, and made an agreed plan to get married to Kirito in the Underworld, unbound by real-world constraints.

As their holiday is ending in a few hours, Kirito agrees to quickly get married at the church in Centoria. The girls then revealed that they all are in the STL running at 10000 times faster, and still have 5 years, 8 months to go.

Trivia

 * It is also titled as "Sword Art Online: Gaiden X2", according to The Assemblage release.
 * Kawahara Reki confirmed on his Twitter that this side story is not canon.