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.

Synopsis from Anime News Network