The Wisdom behind Washing after Sexual Intercourse
Question :
What is the wisdom behind performing Ghusl after sexual intercourse?
Answer:
It is necessary to perform Ghusl after sexual intercourse and after nocturnal emissions, according to what has been mentioned by the Islamic law in this regard, even though the wisdom behind this has not been made clear, because we are required to accept and submit to all the teachings of Islam. And scholars have mentioned the wisdom and benefits in that:
It has been said that it is because it is a major defilement, and so requires washing the whole body, while minor defilement requires the washing of (only parts) of the body. It has also been said that the semen which is discharged from a man causes weakness and laziness and that making Ghusl strengthens him and increases his energy following it. It is apparent that the act of washing strengthens and refreshes the soul after some time has passed, and that abandoning it causes weakness and sickness of the soul and the like.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 96-97