All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Our good religion Islam honors human beings by making lawful an ideal way for them to satisfy their innate desires, and does not permit them to act outside the legal boundaries. Islamic Sharee’ah considers fornication or adultery a great sin. Whoever commits it receives major punishment in Islamic law; if he is married, he could be stoned to death.
This punishment is to protect the society, human lineage and progeny as well. Everything that results through an illegal act has no legal respect in the Sharee’ah. So, there is no relationship to be considered between a man and the child he fathered through illegal sex. Such a child is not traced back to his natural father (to the fornicator), nor do they inherit from each other. Even if civil law considers these relations legal it does not change the reality and does not make it legal in the Sharee’ah. This truth cannot be changed. The following Hadeeth makes clear the subject of tracing a son born from illicit intercourse. "The Prophet decided regarding one who was treated as a member of a family after the death of his father, to whom he was attributed, when the heirs said he was one of them, that if he was the child of a slave-woman whom the father owned when he had intercourse with her, he was included among those who sought his inclusion. But he received none of the inheritance which was previously divided; he, however, received his portion of the inheritance which had not already been divided; but if the father to whom he was attributed had disowned him, he was not joined to the heirs. If he was a child of a slave-woman whom the father did not possess or of a free woman with whom he had illicit intercourse, he was not joined to the heirs and did not inherit even if the one to whom he was attributed is the one who claimed paternity. Since he was a child of fornication whether his mother was free or a slave”. [Ahmad and Abu Daawood] So, he (the father) should keep away from this son and should not deal with him as a son, should not permit him to mix with his daughters, sisters, and wives or to be in seclusion with them.
Furthermore, he (who committed adultery) should not inform anyone of his Zina. Informing others of his disobedience is considered as making it overtly which is prohibited in the Sharee’ah. The Prophet said: “All the sins of my followers will be forgiven except those who commit sins openly or disclose their sins to the people. An example of such disclosure is that a person commits a sin at night and though Allah screens it from the public, he comes in the morning, and says, "O so-and-so, I did such-and-such (evil) deed yesterday', though he spent his night screened by his Lord (none knowing about his sin) and in the morning he removes Allah's screen from himself.” [Al-Bukhari and Muslim]
Finally, the one who has committed a sin should make sincere repentance towards Allah. He does not need to do anything except to turn to Allah in repentance regardless of how many sins he has committed. Allah Says (what means): {Say, “O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.” And return [in repentance] to your Lord and submit to Him before the punishment comes upon you; then you will not be helped.} [Quran 39: 53-54]
Marrying a newly converted girl is permissible if one has made sincere repentance from Zina. The Islamic rulings are common for every Muslim regardless of when he embraces Islam or whether he was born in a Muslim family or a non-Muslim one.
Allah knows best.