Since the sexual instinct is the most difficult to control, one should choose the best way to fulfillment this natural instinct. Taking a moderate stance is liked by Islam and is according to the nature of our creation. Islam does not allow its followers neither exceeding the bounds nor falling short. As an example for opposition of Islam to the theory that has totally abandoned sex, read this story:
A few of the companions of the Noble Prophet of God, decided to avoid having sexual intercourse with their wives, eating breakfast and sleeping at night. Um Salmah (the wife of Prophet) told this to the Prophet. The Prophet Muhammad went to them and asked: “Do you abstain from (going to) your wives? I go to my wives, eat in the daytime and sleep at night. Whoever forsakes my tradition is not from me.” (Wasa’il al-Shi’ah, v.14, p.8)
As you see Islam strongly opposes to those who have considered sex to be an animal act and have called it filthy.
On the other hand, Islam is strongly opposes those who commit adultery. The Holy Qur’an says: “Nor come nigh to adultery: for it is a shameful (deed) and an evil, opening the road (to other evils).” (17:32)
The important point in this verse is that the verse says do not even approach adultery let alone committing. It shows that there are some other deeds that might ultimately lead to adultery. Acts like flirting, looking, nudity, reading sexy books, viewing pornography, and going to centers of corruption or being in a private place with another person of the opposite sex are all preparatory actions which might lead to adultery.
We should remember that making love with our legal wives is the moderate stance regarding sexual instincts.