Well he just had everything he owned burned to a crisp and he didn't like owing people. So he took a job that was a little unsavory, but honest work for the money. He was a bit desperate at the moment and made a fast choice. Maybe not the smartest, but he wasn't trying to deliberatly hurt anyone. So I think we can cut him some slack...
How was he betraying Asakura at all? I mean they were NOT in a relationship, and had no clue Asakura returned his feelings in the slightest other than a drunk kiss. For all he knew he kissed ppl when drunk (IK someone who's like that). So the only thing Asakura had to forgive him about was that he thought he was lying to him, which they cleared up.
To me I was confused since I thought he was going to be an assistant director but turned out to be actor. But to me I wasn't disappointed in him at all. He just lost EVERYTHING, wasn't in a job where he makes a lot to begin with, and needed money. He is a dad that had to make a hard choice to make money in a way he didn't really want to to try and get back on his feet.
Life isn't perfect, neither are people.

A little confused... wasn't Izumi still in the middle of filming the AV? Does Asakura know that there's still SEX scenes coming up in filming? Or was that over and done with? It didn't seem like filming was done just yet. Call me conservative but I don't know how I'd feel if my partner had to film a sex scene with another person after we got together... especially if I wasn't fully aware of it O_O