Hetakoi

For his twentieth birthday, Shizuka chooses to go to a hot springs alone. To his surprise, he also ends up seeing his first naked woman... a passed out drunk girl. Soon afterward he enters college, and his friend from high school convinces him to join the female dominated travel and hot springs club. Amazingly, the girl he saw naked, Ruka, is also a member of the club! Now he must interact with his club more or less normally, and pretend he doesn't imagine Ruka naked every time he sees her. He is determined that his embarrassing secret never be made public!

(Source: MangaHelpers)

Included one-shots:
Volume 5: Katakoi
Volume 10: Toori no Kado wo Magaru made