What is the way to preserve a marriage if the husband accuses his wife of committing indecencies before marriage, based on his viewing of women on pornographic websites whom he believes to be his wife, and linking that to what they committed together before marriage?

You have both done well by repenting, and repentance erases what came before it. Your husband must repent from entering pornographic websites and from his ill thoughts about you. Sincere repentance involves abandoning the sin, regretting it, and resolving never to return to it, as well as avoiding the causes of disobedience. If he does not desist from this, he should be referred to a psychiatrist. We advise you to cooperate with him in obedience and to maintain congregational prayers, remembrance (dhikr), and recitation of the Qur'an; for this brings blessing, tranquility, and affection.