Ruling 2420 — Marriage

If it is established for a man that a woman is married, and he knows it is unlawful to marry a married woman but marries her anyway, he must separate from her and not ever marry her again. If he was ignorant of either matter – i.e. the subject (mawḍūʿ) [the woman being married] or the rule (ḥukm) [of it being unlawful to marry a married woman] – then the marriage contract is invalid but the woman does not become unlawful for him forever, provided they did not have sexual intercourse. If he did not know she was married and had sexual intercourse with her after getting married to her, then based on obligatory precaution, she becomes unlawful for him forever.