Pre-marital sex is unlawful sexual intercourse (zina). Allah forbids not only zina, but even approaching it. He commands:
“And do not approach unlawful sexual intercourse (zina). Indeed, it is an outrageous act and an evil way.” (Al-Isra/ The Night Journey 17:32)
If the fornicators/adulterers do not repent and keep on having unlawful sexual intercourse, they will be led to hell, as well as being forbidden from marriage with decent people. The Glorified Allah commands as follows about fornicators/adulterers :
“A man guilty of adultery shall not marry any but the woman guilty of the same or a polytheist woman, and none shall marry a woman guilty of adultery but the man guilty of the same or a polytheist man. Such marriages are forbidden to true believers.” (An-Noor/ The Light 24:3)
Allah, however, will forgive them if they turn around from their misdeeds, correct themselves and never do it again. The Glorified Allah commands:
“The servants of Allah… And those who do not call for help with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse (zina). And whoever should do that will have sinned.
The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace.
Multiplied for him is the punishment on the Day of Rising, and he will abide therein humiliated;
except for those who turn around (from this sin), trust (in God) and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah The Forgiver and The Merciful.
And he who turns around and does righteous deeds, does indeed turn to Allah with accepted repentance.” (Al-Furqan/ The Proof 25:68-71)
In that case, they can marry chaste believers.