pornography is prohibited in islam. How big a sin is "watching" pornographic material with your wife once in a while?
Watching pornography is forbidden, whether a person is married or not. The one who does that has to repent to Allaah. How can a person watch such things with his wife and he can he permit her to watch it too? Women are weak and are easily influenced; indeed watching such things may lead to problems between the spouses which may end up in divorce – Allaah forbid. Undoubtedly if a man approves of such a thing he is lacking in gheerah (protective jealousy), which is what distinguishes a Muslim from others, from kaafirs and duyooths (cuckolds) who approve of obscenity and immorality in their families. Moreover, watching such material makes one take the matter of immorality lightly and encourages promiscuity. We ask Allaah to keep us safe and sound. Let the one who has committed such a sin beware of the punishment of Allaah; let him hasten to repent. He should be keen to look for anything that may help his wife to remain chaste, not for things that will encourage immoral actions. And Allaah knows best.