Is pornography any less sinful if the actors are married to each other? Is pornography any less sinful if created for the spouses' private use? Why or why not?
Porno, so long as it's between consenting adults, is no more or less sinful than sex.
If you consider sex as a private matter and you are religious, the concept of sin creeps in (or rather kicks the door down and takes control). If not, it's not and is seen as what it should be - a natural bodily function. (And not just for procreation either - if it were just for making babies, women wouldn't orgasm)