Can you blame Adam? SPOILERS
First of all, Joan and Adam were probably one of the most dysfunctional TV couples I've ever seen. Combine that with her weird and flaky behavior, her insecurity, her jealousy over stupid things like a hug and a cute assistant, as well as her kissing Roger. Not to mention her not wanting to have sex even after over A YEAR together. Granted, Joan was a virgin and still in high school. So it's not like it was two adults waiting a year to have sex. But still...
Now I will say that he should have broken up with her before doing anything with Bonnie. Cheating is wrong. However, I don't blame Adam for wanting to stray. Even if Joan wasn't frigid, she was still a mess to be with IMO.