My feelings exactly!
Work is still work. He already explained it was his work before, and they weren't together the time it happened so I can't see what's the problem.
What bothers me is that they COULD HAVE BEEN together, if pride was set aside and blinders off to the fact that Asakura was obviously more than open minded to having a relationship. Ken didn't want to get back into the industry because of Motoki and only did it for the money so that he could move out of Asakura's place, which he didn't need to do if he listened to Asakura's pleas, and openly confessed to Asakura in the first place. No AV needed.
But it was not unnecessary (discounting the fact that it was a way to put into the lights to Asakura that he wanted to have something with Ken), the AV opened new jobs for Ken, like the theater acting which was something he wanted to do. Thing was, it was not as shown because the romantic side was clearly what was important. I believe it was nice to put that, even though I'd have wanted to see more that side of him. Pride showing and misunderstandings/jealousy are almost a standard on many BL works, and this time it was managed rather nicely I believe.
I loved, loved this story at first, but then Ken filming an AV ruined it for me. He said he needed money, but Asakura had been insisting that Ken rely on him for Motoki's sake, and if Ken had just swallowed his pride about the money, sucked it up, and confessed to Asakura (I mean, really, it was obvious Asakura liked him and he totally had a chance), then he wouldn't have had to do the AV. And Ken's "oh yeah, he's not my lover, we only filmed an AV ALL DAY yesterday together, it's no big deal, I needed the money" attitude, along with Asakura's "oh gee, I feel so much better now that you told me that" light-hearted response is just too easy. Asakura should have been like, "WTF?! You didn't need to do that you dumbshit!" I'm dumbfounded. The manga was good enough (cuteness, tension, hardship) without that turn of events.