I agree with the "often" part. And it is certainly a complicated dynamic. I believe that most of the time, both spouses have a role to play (albeit not necessarily an equal one). And most of the time, affairs tend to be more symptoms than causes, that is a reflection of problems in the relationship rather than the cause of the problems themselves.
Also, don't forget the sex addict, which is a whole other thing entirely. I suspect that when dealing with a sex addict, nothing but a consequence-based approach would likely stand a chance of being effective (and even then, the odds are not that great, imo).
