There are two different types of unlawful sexual intercourse in Islam: fornication and adultery. Fornication is the unlawful sexual intercourse of someone who is not married and was never married before. The punishment for fornication is getting flogged 100 lashes. Adultery is unlawful sexual intercourse of someone who is married or was previously married before. The punishment for adultery is to be stoned to death, and it is actually the only sin in Islam that is punishable by death by way of stoning.
Now how is this justifiable in Islam? What people don’t realize is that the criteria for this punishment to be set in place is incredibly strict. First of all, there has to be four, practicing, male Muslims who witnessed the physical act of penetration. It doesn’t matter if the witnesses saw them walk into each others homes, or if they were seen together talking to each other, or even if text messages were shown. Rather, the actual act of penetration has to be witnessed by those four witnesses. Then those four witnesses would have to go and testify against them in an Islamic court. When put into perspective, this criteria is essentially impossible to be met unless the people decide to have intercourse in public. The other way this punishment can take place is if the adulterers go and testify against themselves. Therefore, one can see that some of the reasons that Allah puts these punishments in place is as a deterrent and to protect the people, but as for the criteria that has to be met, it is extremely strict. People don’t realize the gravity of the crime that they’re doing, as this doesn’t only affect the two individuals, but rather the whole family structure can be quickly broken down and this destroys societies from within. This is why one can see that even according to the Bible, adultery is also punishable by death (Leviticus 20:10). Now imagine if people lived in a world where no one cheated on each other and no one would be disloyal to their partner; how much better would society be as a whole?
And Allah Knows best.