The Holy Quran says,
And marry those among you who are single and those who are fit among your male slaves and your female slaves; if they are needy, Allah will make them free from want out of His grace; and Allah is Ample-giving, Knowing.
The above ayat begins with the words Wa Ankehoo (And marry…) The imperative form of the word 'nikah' implies that either it is obligatory or highly recommended. According to scholars, though marriage is a highly recommended act, it becomes obligatory when there is a chance of falling into sin.
On another occasion the Prophet (s.a.) said,
“The best people of my nation (Ummat) are those who get married and have chosen their wives, and the worst people of my nation are those who have kept away from marriage and are passing their lives as bachelors.” 
Imam 'Ali (a.s.) exhorts, “Marry, because marriage is the tradition of the Prophet.” The Prophet (s.a.) also said, ”Whosoever likes to follow my tradition, then he should know that marriage is from my tradition.” 
A. Importance of sex in marriage
In Islam, marriage is not restricted to a platonic relationship between husband and wife, nor is it solely for procreation. The Islamic term for marriage, “nikah” literally means sexual intercourse.3
So why has Islam provided extensive rules and regulation regarding sex? This was because Islam has fully understood that sexual instincts cannot and must not be repressed. They can only be regulated for the well being of human beings in this life and for their success in the hereafter.
B. Fulfillment of Sexual Urge
The Holy Prophet (s.a.w.s.) and the Holy Imams (a.s.) also encouraged their followers to marry and to fulfill their sexual urges in lawful ways as can be seen from the following: The Prophet (s.a.) said, “O you young men! I recommend marriage to you.” 
C. Celibacy and Monasticism is Forbidden
Islamic is totally opposed to monasticism and celibacy. 'Uthman bin Maz'un was a close companion of the Prophet. One day his wife came to the Prophet and complained, “O Messenger of God! 'Uthman fasts during the day and stands for prayers during the night.” In other words, she meant to say that her husband was avoiding sexual relations during the night as well as the day. The Prophet was angered. He did not even wait to put on his slippers. He went to 'Uthman's house and found him praying. When 'Uthman finished his prayers and turned towards the Prophet, he said, “O 'Uthman! Allah did not send me for monasticism, rather He sent me with a simple and straight [Shariah]. I fast, pray and also have intimate relations with my wife. So whosoever likes my tradition, then he should follow it; and marriage is one of my traditions.” 
D. Beneficial Effects of a Married Life
Various studies prove that married people remain healthier, physically and mentally. Islam, has always maintained that marriage is beneficial for us in many ways.
Islam also regards marriage as a way to acquire spiritual perfection.
The Prophet (s.a.) said, “One who marries, has already guarded half of his religion, therefore he should fear Allah for the other half.”  How true! A person who fulfills his sexual urges lawfully would rarely be distracted in spiritual pursuits.
E. Marriage enhances the value of prayers
A woman came to the Prophet (s.a.) and said that she had tried everything to attract her husband but in vain; he does not leave his meditation to pay any attention to her.
The Prophet (s.a.) told her to inform her husband about the reward of sexual intercourse which he described as follows: “When a man approaches his wife, he is guarded by two angels and [at that moment in Allah's views] he is like a warrior fighting for the cause of Allah. When he has intercourse with her, his sins fall like the leaves of the tree [in fall season]. When he performs the major ablution, he is cleansed from sins.” 
F. Marriage increases Sustenance
The Holy Prophet (s.a.w.s.) remarked, “Give spouses to your single ones, because Allah makes their morality better (improves it) (under the shadow of marriage) and expands their sustenance and increases their generosity (human values).” 
 Surah Nur 24: 32
 Marriage and Morals in Islam, Sayyid Muhammad Rizvi
 Wasaelush Shia, vol. 14, p. 3
 Mustadrakul Wasael, Muhaddith Noori, vol. 2, p. 531 quoted in A Gift for the Youth, Shabeeb Rizvi
 Wasaelush Shia, vol. 14, p. 3-4, 6
 Surah Baqarah 2:222
 Wasaelush Shia, vol. 14, p. 25
 Wasaelush Shia, Vol. 14, p. 4
 Wasaelush Shia, Vol. 14, p. 10
 Wasaelush Shia, Vol. 14, p. 5
 Wasaelush Shia, Vol. 14, p. 7
 Wasaelush Shia, Vol. 14, p. 74
 Nawadir al Rawandi, p. 36