If you liked the Koibana Onsen manga, the Anime-Planet community thinks you'd like:
Daikichi is the epitome of unlucky. He's just lost his job, and what's worse, he unknowingly moves into an apartment where a girl once committed suicide. Though as luck would have it, he discovers that she's still floating around, and is a lovely, kind ghost – the kind of ghost that Daikichi quickly falls in love with, physical body or not. Together, the two live day to day, having sex at every opportunity, and slowly falling deeper and deeper into love.
While Itoshi does have a romance subplot (and I say subplot given it's definitely secondary to the nonstop sex), both of these manga focus almost entirely on explicit sex. So if you liked it in one, read the other.
Urashima Keitarou is a loser. He's failed the Tokyo University entrance exams two years in a row, and his parents have finally kicked him out of the house. Penniless and destitute, he travels to his grandmother Hinata's inn, hoping she'll put him up until he can get into Tokyo U and fulfill his promise with his childhood sweetheart (whose face he can't even remember). Unfortunately - or perhaps fortunately - it turns out that the inn is no longer an inn... it's an all-girls dormitory, as he discovers in his typical blundering fashion by grabbing an eyeful and more of every girl in the place! Narusegawa Naru, a fiery-tempered high school student and the first victim of Keitarou's lewd bungling, wastes no time doling out brutal punishment and preparing to kick him back out to the street. But wait! A letter has come from grandma Hinata, and Keitarou is to be the dorm's new live-in manager! Is this a blessing from heaven, or has he been cursed to a living hell?
Some of the similarities are striking: a lonely, clueless guy moves into a hotsprings resort/hotel after various improbable circumstances and begins life with several beautiful women who invariably fall in love with him, for whatever reason... and the hijinks ensue. Although Koibana Onsen is *much* more explicit than Love Hina and intended for a more mature audience, some of the same themes are there, namely, first love, lots of humor, misunderstandings, and so on. Love Hina has plenty of suggestive content, but is still fairly innocent in its subject matter, while Koibana Onsen emphasizes one thing above all others: sex, and lots of it. I wouldn't expect too much out of either series if you are looking for any depth to the plot, meaningful dialogue, or a credible romance. However, if you check your brain at the door, both series offer some ecchi-themed laughs.
Hideo Aiba is anything but popular. Not only is he a high school virgin, he's also a part of the "Roman Club" – a group of guys with bizarre fetishes. But Hideo's unusual school life is changed forever when the mysterious Kurumi transfers into his class, joining the Roman Cub soon thereafter. Together, the two begin a strained and pleasurable relationship where Kurumi controls Hideo's very desires... and actions.
While performing a delivery to a beautiful woman’s apartment one night, twenty-five-year-old Kimijima finds himself in the arms – and loins – of his customer, the newly-divorced Kana-san. The next morning Kimijima heads to an interview, only to find that one of his interviewers is Kana-san herself! Now, Kimijima must work hard at his new job while having ample sexual encounters with not only Kana, but hostesses, fellow interviewees and old friends (at least in his dreams) - to say the least! However, to make things more complicated, Kimijima finds himself falling for Kana...
Koibana Onsen and Virgin na Kankei are two manga that likely will only be enjoyed by those who want hentai-level sexual encounters (as both are completely filled with them). There's an underlying romance plot and often a variety of girls to choose from, but ultimately both boil down to the same thing: graphic, explicit sex.