There would be no problem for a man to get married to a woman whom he has already committed adultery with if she had no husband or got divorced but passed her Iddah (waiting period) completely on that time. Imam Khomeini (r.a) said in this regard: "If a man commits adultery with a woman who has no husband or gets divorced and passes her Iddah completely he will be able to get married to her. However, he is supposed to wait till she menstruates then conclude the marriage contract with her, according to the mustahab (recommended) precaution.[1]
Otherwise, if the man commits adultery with a woman who has a husband she will become haram forever to the adulterer. This means, he can never get married to her after she gets divorced from her husband or her husband passes away[2].[3]
[1] . Imam Khomeini, Tawzih al-masa'il (al-mohasha), vol 2, p 470, question 2399.
[2] . Tahrir ul-Wasilah, vol 2, p 251, question 22, published by Ja'miah -e- Mudarresin; Imam Khomeini's Istiftaat, vol 3, p 297. Imam Khomeini (May his soul rest in peace) said: "If one commits adultery with a woman who has a husband she will become haram forever to the adulterer (meaning that he can never get married to her, even if she ever gets divorced from her husband) and there would be no difference between whether the adulterer knows that she has a husband or doesn't, also, between the adulterer concludes the marriage contract with her or not. The same ruling is also applied to a woman who gets Rejeiah divorced. It should be noted that other marja' have the same ruling in this regards. However, according to Ayatollah Tabrizi, seemingly, she won't become haram to the adulterer (meaning that he can get married to her) (Tawzih al-Masail, vol 2, p 472, question 2403)"
[3] . Adapted from answer 909 (website code: 994), Index: getting married to a woman whom the man committed adultery with.
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