Torotoro Hitou de Koi, Hajimaru.

Iharu is a university student looking for ways to kill time over spring break. In passing, he meets an attractive young man named Shin who invites him on a trip to the hot springs. He accepts Shin's offer and finds himself at an elegant inn with outdoor baths and hidden springs—not to mention luxurious meals and amazing sake... Lost in his happiness, he decides to enter the hot springs with Shin. But when he looks at the young man undressing, he can't believe his eyes... "Even his dick is gorgeous! His butt is so shapely!" Even after Iharu turns in for the night, he can't stop thinking about Shin's amazing body... and growing increasingly horny.

(Source: Torico)