I believe that Phun knows that. Part of the struggle from the beginning is that Phun cares about Aim and Noh cares about Yuri.
Luckily Noh is more up front with Yuri but Phun and Aim have already been intimate. Phun does not want to hurt Aim but he is realizing that loves Noh more deeply and is struggling with how to tell her. We see this is his interactions with her after Phun and Noh get together. But just becaue he loves another doesn't mean it is not hurt. We also have to take in consideration that Phun loves Noh but Aim is messing around with multiple guys and it is not out of love. Should Phun have broken up with Aim. Probably. But he is not sure about his feelings and Noh feelings until recently. And before he hurt her he wanted to make sure. In the novel we don't know what Phun says to Aim. We only know he talks to her. Even then I doubt if he told her about Noh. We dont know why Aim cheats on Phun. But we do know why Phun is with Noh. He loves him more than Aim.