Hetakoi
Thankfully, this one was pretty good! The plot it's started when a guy in his 20th birthday goes to a hotspring alone and finds a woman passed out on the other side. Since he was embarrassed about seeing her in that state (she was totally spread open, so to speak), he just left after calling the personnel of the inn. He thought that it would be the end of it, but an old senpai of him called him in college and suggested going to the Tabisen club (a club dedicated to the hot springs culture) and finds that one of the members in that club is the same woman he saw passed out, after she speaks about some details of her last vacations. The plot it's essentially a simple love story with plenty of drama involved and terrible coincidences all around. Thankfully, the end it's pretty good and and pretty satisfactory, so it's something that I would recommend if you're looking for something romantic with some amount of sexiness (the series contains some nudity and depictions of sexual acts but nothing too blatant).
Sadly, I read that the mangaka that made this manga died in 2013 due to a heart disease. At least it seems that she didn't left any series orphaned, but it's a shame that she passed away