All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
You are required to do nothing if you had married that man while forgetting all about that oath or being unaware that he was the very man whom you swore not to marry.
Shaykh Al-Islam Zakariyya Al-Ansaari who belongs to the Shaafi‘i school of Fiqh, said, "If a person takes an oath and then forgets all about it, or ignorantly did what he swore not to do, or is forced to do that act, his oath is not considered broken. This applies to taking an oath by Allaah, or an oath of divorce, or swears an oath to free a slave. This is because the Prophet, sallallaahu ‘alayhi wa sallam, said: "Verily, Allaah Has Forgiven my nation for mistakes and forgetfulness, and that which they are forced to do under duress." [Ibn Maajah, Ibn Hibaan, Ad-Daaraqutni, At-Tabaraani, Al-Bayhaqi, and An-Nawawi: Hasan]
Allaah Knows best.