QUESTION:Dear scholars, As-Salamu `alaykum. What is the significance of making ghusl (bathing the whole body) after sexual intercourse? Jazakum Allah khayran.
ANSWER:Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister in Islam, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake.
First and foremost, we would like to stress the fact that Islam is the religion of cleanliness. Islam requires a person to pay considerable attention to his personal hygiene and cleanliness. The Qur’an says: "Truly Allah loves those who turn unto Him, and loves those who have a care for cleanness." (Al-Baqarah: 222) The Prophet (peace and blessings be upon him) states: "Cleanliness is half of faith."
In his response to the question you raised, Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states:
"This ghusl is called ghusl al-janabah according to the Shari`ah. It is obligatory to take a full bath after sexual intercourse or ejaculation (i.e. in a wet dream or if sperm comes out due to sexual desire, also sexual dream and discharge in the case of women). This is mentioned in the Qur’an and the Sunnah. The purpose of this bath is to clean the body and refresh the spirit. Sexual emission is not just an ordinary matter like urination or defecation. When it happens, the whole body, mind and spirit get involved in it. After it, one feels lazy and dull. The full shower makes a person fresh in body and spirit. There are many spiritual as well as physical and medical benefits in it. But for a Muslim, it is the command of Allah and His Messenger and it must be followed."
Excerpted, with slight modifications, from: http://pakistanlink.com