That's what I find it annoying. I felt betrayed! Why would he sleep with someone else if he's really inlove with Akihiko? Plus they didn't make it clear to us. I hate the author for doing that! It could've been great of they didn't add the "cheating" part. I HATE CHEATING!
A question: It became clear that Akihikodidn't sleep with Naoyuki, but it seems that Kyoujima DID sleep with that actor, right?