I'd contend that couples who feel the need to add porn to their relationship are doing so to seek out a form of escapism - nevermind the fact that they're watching fornication, committing mental adultry etc...
I'll contend that Porn can easily sneak in and cheapen the sexual side of marriage because it paints a picture that's not real, can cause depression in one or both of the spouses if the other doesn't seem to be "as adequate" as the person on-screen...
We, as a culture love to medicate our minds with things that aren't real... Why do you think porn, video games and TV are such huge industries?
Rather than focus ALL of our attention on our spouse - whom we vowed to love, cherish, honor and respect, many couples say "oh, it's okay to add porn because that helps spice things up"...
If you need spices go look in your kitchen cabinet....
If you want good sex, make it your mission to tend to your spouses emotional, physical and spiritual needs..
Sex is a spiritual experience, and there's no better way to have awesome sex than when both people have a proper relationship with their maker. Why? Because when you're filled with perfect love, the love Corinthians talks about, you'll have no selfish desires whatsoever....
I'll contend that many people are very selfish when it comes to sexual fullfillment.
Next time you want to add porn to your sexual time, instead, try adding a dose of complete unselfishness to it and see what happens!