Tsukishima and Yamaguchi have always been best friends, but for all that Yamaguchi adores and idolizes the other boy, he himself is not attracted to men. Tsukishima is, though; and eventually he reveals to Yamaguchi that he loves him and wants him badly. Yamaguchi, though unhappy, loves Tsuki so much as a friend (platonically) that he's unable to refuse him when he asks for sex and/or a physical relationship.

Bonus if Tsukishima knows that Yamaguchi's totally undesiring of his advances and feels remorse over it, but instead of making himself stop, Tsuki instead resolves that he'll become good enough at pleasuring Yamaguchi in bed so that the other boy eventually starts enjoying it and wanting more despite himself. (Working in Karasuno/other players into the story is also love!)