Natsu e no Tobira

Marion is a young schoolboy who prides himself on his adherence to a philosophy he calls "rationalism". Because of his disdain for emotional display, he ignores anything remotely akin to affection. But when he's entangled in a romantic affair with an older courtesan, his rationalism is revealed to be little more than a cover for his own emotional immaturity. Learning to love, Marion blossoms under his older lover's care but unfortunately, Marion has yet to learn the true price of the affair.

(Source: Anime News Network)