Why Zina is prohibited
2-12-2002 | IslamWeb
Question:
Why is it wrong to have sex outside marriage?
Answer:
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.
Allah has forbidden Zina (fornication or adultery) and made committing it a major sin that is punished severely. Allah Says (what means): {… nor commit illegal sexual intercourse and whoever does this shall receive the punishment.} [Quran 25: 68]
Allah not only forbids it but also forbids all ways that lead to it. Allah Says (what means): {Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts, etc.)…} [Quran 24: 30]
Islam has set a very deterring punishment for committing such an act. So, the virgin is given hundred stokes and exiled for a year while the married, divorced or widowed is stoned to death if committing Zina.
Allah Says (what means): {The woman and the man guilty of illegal sexual intercourse, flog each of them with a hundred stripes…} [Quran 24: 2]
There are many reasons for forbidding Zina. Here are some wisdom behind this great ruling:
1. To avoid intermixing of blood kinship as this leads to losing track of progeny and neglecting rights. In fact, the society is but individuals that constitute families. So, if these families got lost and the responsibilities can no longer be defined, then people become like animals.
2. Zina is a disgrace to the personality, the honor and the dignity of the person. It is also a big injustice towards the woman as it forces her to raise up children on her own.
3. Zina causes a great number of diseases that have left doctors puzzled and the scientists unable to act.
We remind you here that Islam does not forbid anything except that it is harmful, and it always gives better alternatives. To Zina the alternative is marriage. To Riba (interest and/or usury), the alternative is selling and buying …etc.
Allah Knows best.