Tonari no Wakazo

Araki, a gay man who takes his relationships seriously, doesn't like younger men. He prefers older men who are more faithful. And yet, his new neighbor is a young, flashy-looking college student named Higata, whom Araki finds annoying to deal with. Until one day, he is suddenly kissed by Higata, who says, "I feel relaxed when I am with you,” Araki can feel his heart pounding, even though he's totally not his type!

(Source: Kadokawa, translated)

Note: Includes one extra chapter and the one-shot "1 day Contact (ワンデーコンタクト)".