Well, they have just said they both want to, but haven't really agreed yet... I feel a "but" coming. Maybe something to do with the talent agency, or college.
Seto thinking about that kiss with Izumi is not good. He's gonna be confused and maybe do something stupid. He probably feels like he betrayed Iori somehow, but he really didn't know at the time. I hope he doesn't do something stupid and/or painful to try and make it up to Iori...
Just let it go, and on the phone as agreed, tell Izumi no to being BF/GF.