Hayato Suo being romantically involved anyone sounds very foreign to Sakura's and Nirei. It's not that Suo doesn't seem ready or fit, he's obviously more than capable of holding a relationship. The only reason why Nirei and Sakura cannot imagine is because Suo isn't the type to get romantic feelings for anyone. You can only imagine how Sakura's and Nirei's faces looked when they walked in on a girl confessing love to Suo.

??? (Girl): "I... I really love you, Suo!"

Sakura's eyes widened, he quickly flushed red from a distance. Sakura's face always gets intensely red whenever he hears anything involving 'love'. Nirei is very shock, more surprised than Sakura Thats someone likes Suo.

Unlike his friends, Suo isn't surprised. Suo known the girl liked him. He just never said anything until she decided to admit it.

Suo: "I apologize but I cannot accept that because I'm already dating someone."

Suo isn't gonna give her false hope. The never-ending, charismatic smile is glued to his face. It doesn't leave even when rejecting her. He won't show any real emotions.

Sakura, Nirei, and that girl are all beyond shock to hear Suo is a relationship. Sakura and Nirei just don't express it since Suo doesn't need to realize how long they've been standing where they are.

??? (Girl): "Y-You're... taken...?"