What is the ruling on watching pornography once a month due to the husband not granting the legitimate right?

Watching these movies is a reprehensible act that corrupts the heart and distances one from Allah; they are forbidden. The husband's failure to provide chastity is not a justification for watching forbidden things. Rather, one should communicate and be frank with him, stating that it is the wife's right to be given chastity. If the husband is ill, he should seek treatment. If that does not help, and the wife is able to be patient and remain chaste, then so be it. Otherwise, she has the right to seek divorce or annulment (khul').