There are several presumptions in this thread:
Porn is bad.
Is it, necessarily? Is it a bad thing if two people who love each other (or three people?) use pornography to stimulate their sex lives? The assumption by many people in this thread is that porn is a bad thing with no redeeming value, destabilizing to relationships, and causing people to waste time and energy chasing fantasies that can have no basis in reality. Yes, it can be those things, but by no means must it be those things. It can be a valid part of a healthy relationship, and it can be an excellent way for a single person to relieve stress.
Sin.
To accept the concept of Sin, one must first acknowledge the concept of a Judeo-Christian monotheistic God. Personally, I feel that the very concept of Sin is equally as ridiculous as the very concept of "worshipping" God. In looking at porn without the concept of God, one should ask, "is doing this harming me or anyone I care about? What are the benefits?" No higher powers needed, thank you.
Marriage.
Marriage is an human cultural artifact, not a law of nature.