Well...
I'm hesitant to jump in here, because this is such a personal issue for people. I also realize that while I view this through a religious/spiritual lens personally, that lens does not apply to every other person.
But, let me say this in relationship to marriage and pornography...in general. I will let others sort out the whole "sin" issue as an aside.
Let me start with women, since I am one. ;-).
Women as a whole respond emotionally to sex differently than men, there is no doubt. What we end up doing to "cheat on our spouse" if you will, is develop emotional relationships with others, or perhaps complain incessantly to our friends about our spouses, or to withhold sex as a weapon to negotiate more favorable treatment/secure favors. Not one of those things is healthy, and every single one of them creates pain for our spouse.
Men respond to sex physically. As a result, pornography or quick, easy sex and a wandering eye are huge temptations. Men may "cheat on their spouse" by objectifying sex as simply a physical act, and disregarding any pain they may cause their wives. In other words, "It didn't mean anything to me, so it shouldn't mean anything to you." If a woman is simply a servant/prostitute for a man, and is ignored emotionally, he is "cheating on her"...even if he never actually has sex with another person. Not one of those things is healthy, and every single one of them creates pain for a spouse.
The fundamental truth here, is that our roles as husbands and wives is to go out of our way to not cause pain for the person we have promised to love, honor and cherish. Even if it seems crazy to us.
I guess the bottom line is that sexual issues are generally a symptom, rather than a cause, of marital strife. Different people may organize the details of this relationship differently, but in the end, both partner's needs and emotions need to be respected. That is why some women don't worry about pornography, while it is extremely painful for others. And why some men don't mind their wives having platonic male friends, and other's do.
I am not completely sure this post makes sense, because I am trying to avoid a LOT of potholes...But at least it is a start to a more sane debate on the subject.
If that is really what people want to do on this thread.