There is no justification for sin, period.
Viewing porn is one sin among many that damage a marriage. It is stereotypically a male problem (though is increasingly common among females), and it is in this context that I'm addressing it. My comments regarding males and females are generalities, and as such, there are always exceptions.
The sin that men commit, such as physical violence and viewing of porn, does damage to their wives, and I'm not arguing that. However, the sin that women commit, such as emotional adultery, manipulation, disrespect, and refusing sex, do damage to the same degree to their husbands. Such behaviour will tear holes in his confidence, identity, and self-worth, and will push him away from you. It is exactly the opposite of the edifying behaviour that would strengthen the marriage.
If you as a wife are watching soap operas, reading romance novels, and "going out with the girls", are you any better than the husband who looks at porn? Are you not looking for something in others that you don't find in your husband?
When we consider the sin of the other person as "worse" than our own, we basically justify or excuse our own; that is not repentance, and it certainly won't heal a damaged marriage. Better that we each look to Jesus, measure ourselves against His standard of righteousness, and seek His help to treat our spouse as He desires.