I would imagine Nishinoya being quite flexible because he’s so short and therefore might have some natural flexibility. :D
During warm and cool down stretches Nishinoya is eyeing Asahi from across the room. He’ll be doing a hamstring stretch or a quad stretch and he’s bending all the way down. Nishinoya notices Asahi is looking at him from across the room and he starts winking and blowing kisses at him teasingly. Asahi can feel arousal beginning to pick at him and he’s hoping no one notices the boner that is slowly starting to take shape under his pants.
After the cool down stretches, Asahi ‘borrows’ Nishinoya and he drags him outside. They make out for a little bit before returning back into the gym.