Sperm is Pure
Question :
I had sex with my wife, then I got up and made Ghusl and prayed the Fajr prayer. Is it permissible for me to return to sleep on the same mattress on which I had sex with my wife, and cover myself with the same cover?
Answer:
Semen is clean, and does not make a person or his mattress impure. 'A'ishah, may Allah be pleased with her , said: "I used to rub it (i.e. sperm) from the clothes of the Prophet, then he would pray in them."
However, this is if the semen is emitted after the person has washed his private parts with water (Istinja') or stones (Istijmar) in accordance with the Islamic law, to cleanse himself after urinating. But if it was not so, the semen will not remain pure due to coming into contact with a place which is impure, and thus it will become impure.
And based upon the fact that sperm is clean, it is permissible for a man to sleep on the mattress and to cover himself with the covering under which he had sex with his wife, and there is no harm in that.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 87-88