I see a lot of criticism of Kayla for her "role" in all this, but let's keep a reality check on what happened here, really. Nick and Kayla had a good marriage and a good family. It was Nick that went into the gutter of the Internet and once we was reprogrammed to think "yeah you know there really isn't anything wrong with foul language, aggressive postering, speaking 'my mind', sex and multiple partners and drugs", he then sought to bring her down to his level - all the while thinking this new thinking was actually elevated thinking.
This degraded behavior, amoral approach is merely the drug that is offered by the gutter of the Internet, and Nick got addicted and then he dragged her into it.
Like with all addictions, Nick had a good time, made money and this was "evidence" that the "new way of life" must be "OK and it is working", but like all cons, addictions etc. they always have short term gains, until eventually the behavior shows the true cost.
I blame Nick rightly for dragging his family down, he knew the immoral choices he was making and that they went against his own code of conduct, and did it anyways. A code of conduct is supposed to be the stop gap that prevents one from falling into these situations - it is not a measure of happiness, (i.e. happiness is not the indicator if a choice is right - the code is) because even if it feels good at the time, or seems beneficial, the code let's you know you have wandered.
These fools like Nick simply have no discipline. If they had discipline, they would not fall into these traps. Men easily fall into the lust and satisfaction of our primal urges, and Nick is no different from any man in this regard, where he differs is that he has no discipline.