The consequences or punishment in the light of Islam for those who have committed illegal sexual intercourse is to stone them to death if the culprit person so is married already. But if a person who has done so is not a married person then, he would be punish to get 100 lashes. Here the rule of stoning a person to death penalty is not a new one in Islam. Basically Islam is the continuity of old monotheistic religions. I.e Judaism and Christianity. So there are some punishment which are not given in Qur'an but they were given in old religious scriptures which are Old testament of Bible. I.e Bible for Judaism. Which is called "Torah" in Arabic. And the Christian Bible which is called "Injeel"in Arabic. So that law was taken from Torah. And it is not mentioned in Qur'an. But basically it is mentioned in the sayings of Holy Prophet that is called Hadith. While the other punishment which is mentioned for 2 persons who are not yet married. That punishment is mentioned in Qur'an in the chapter named Surah Noor. And for more understanding. If 2 person have committed an illegal sexual intercourse. And one of them is married and the other is unmarried. Than married will get the punishment which is described for the married person. I.e stone to death. While the non-married will get hundred lashes. There are many rules to punish the criminals. Because Islam does not like to be unjust about anyone.