Firefly Catching (蛍狩り)
Firefly Catching (蛍狩り) is a story written by natsumachi (馬遅夏海) and illustrated by coffee_flowers. It ran in Issue 45 and can be read at LJ and the S2B2 website. Also available on AO3 with content tags.
A historical fantasy romp about a festival night, running around in masks and having anonymous sex in the bushes.
"As a gimmick, many of the districts' entertainment houses ordered large amounts of fireflies to release in their gardens and party rooms. But since everyone was doing it, it had lost its charm. Even the low folk making their homes in a riverbed could enjoy the view of fireflies released near the K River. At night, during the start of the festival season, all mingled. Groups of children, yelling and running along the riverside through willows and cherries, chanted the firefly-hunting songs, though there was no sweet water in the capital. Surely it was also tiresome for the prostitute boys dressed up in pale greens and yellows for the occasion. There were even rumours of gold dust glowing on buttocks in shows of excess among patrons."
From fail-fandomanon.dreamwidth.org: "Set in Japan in festival season. A young man, masked, encounters with a masked tengu (a type of youkai); there is blindfolding and sex."
Firefly Catching has no connecting stories published in S2B2, but it is part of the TAO 'verse which is quite episodic and has no particular starting point (the writer realizes how confusing and frustrating that is, but there's no solution yet). Akiku is one of the main characters of the 'verse and has one of the highest character tag counts.