If a Muslim cannot marry soon after puberty, then he or she just has two options: temporary abstinence or temporary marriage.
(A) Temporary Abstinence
Islam has allowed marriage as soon as a person becomes physically mature, and it also strongly recommends that at least during the early years of marriage to adopt a simple life-style so that the lack or paucity of financial resources does not obstruct a happy life.
But if a person decides, for whatever reason, not to marry soon after he or she becomes physically mature, then the only way is to adopt temporary abstinence. After strongly recommending the marriage of single people, the Qur'an says, "And those who cannot marry should practice restrain (or abstinence) till Allah enriches them out of His bounty." (24:33)
However, abstinence from all the forbidden ways of fulfilling the sexual urge is not easy. Therefore, a few guide-lines would not be out of place. Once a man came to the Prophet and said, "I do not have the (financial) ability to marry; therefore, I have come to complain about my singleness." The Prophet advised him how to control his sexual urge by saying, "Leave the hair of
your body and fast continuously." (Wasa'il, vol. 14, p. 178) By saying that "leave the hair of your body," the Prophet is asking not to remove the hair which grows on pubic area, chest, etc, by shaving or using lotion or wax; rather one should just trim the hair.
This hadith is indicating that removing the excessive hair increases one's sexual urge. (Probably, that is why the shari'ah has recommended the men to shave the excessive hair every forty days, and the women to remove the excessive hair by lotion or cream every twenty days.) In retrospect, it means that not removing the hair will decrease the sexual desire and help the person in abstinence. Imam 'Ali says, "Whenever a person's hair increases, his sexual desires have also decreased." (Wasa'il, vol. 14, p. 178) I have not yet come across any scientific discussion on the relationship between removing of the hair and sexual urge, but I am told that the hakims believed that removing the hair from the pubic area increased the chances of direct pressure on that area and, consequently, the blood flow to the sexual organs.
The other method of decreasing the sexual urge is fasting. It is obvious that one of the greatest benefits of fasting is the strengthening of one's will power. And no doubt, abstinence in the sexual context mostly depends on the will-power of the person. So fasting will strengthen the will-power of the person and make it easier for him or her to restrain the sexual feelings.
(B) Temporary Marriage (Mut 'a)
If a person does not marry soon after maturing and finds it difficult to control his or her sexual desire, then the only way to fulfill the sexual desire is mut'a.
In Islamic laws, according to the Shi'ah fiqh, marriage is of two types: da'im, permanent and munqati', temporary. The munqati' marriage is also known as mut'a. This is not the place to discuss the legality or the illegality of the temporary marriage (mut'a). It will suffice to say that even according to Sunni sources, mut'a was allowed in Islam till the early days of the caliphate of 'Umar ibn al-Khattab. It was in the latter period of his rule that 'Umar declared mut'a as haram. It goes without saying that a decision by 'Umar has no value in front of the Qur'an and the sunnah!
As for the relevance of the mut'a system in modern times, I will just quote what Sachiko Murata, a Japanese scholar, wrote in her thesis on this subject: "Let me only remark that the modern West has not come near to solving all the legal problems that have grown up because of relatively free sexual relationships in contemporary society. If any real solution to these problems is possible, perhaps a certain inspiration may be drawn from a legal system such as mut'a which, with its realistic appraisal of human nature, has been able to provide for the rights and responsibilities of all parties." (Murata, Temporary Marriage in Islamic Laws (Qum: Ansariyan, 1991) p. 4. For a detailed discussion on the social aspect of mut'a, see Mutahhari, The Rights of Women in Islam (Tehran: WOFIS, 1981) and on the legal aspect, see Kashifu 'l-Ghita', The Origin of Shi'ite Islam and Its Principles (Qum: Ansariyan). For an in depth study on the Qur'anic verse and ahadith of mut'a, see at-Tabataba'i al-Mizan, vol. 8 (English translation) pp. 130-161.)
The main difference between the two types of marriage is that in permanent marriage, Islam has clearly defined the duties and obligations between the spouses. For example, it is the duty of the husband to provide the basic necessities of life for his wife and the wife is expected to not refuse sexual relations without any religious or medical reason. But in temporary marriage, Islam has given the prospective spouses the right of working out their own duties and expectation plans. For example, the husband is not obliged to maintain the wife unless it has been so stipulated in the marriage contract. Likewise, the wife can put a condition in the marriage contract that there will be no sexual relations. (Al-Khui, Minhaj, vol. 2, p. 267) Such conditions are invalid in a permanent marriage but allowed in temporary marriage.
I cannot overemphasize the temporary nature of mut'a. The message of Islam is quite clear: marry on a permanent basis; if that is not possible, then adopt temporary abstinence; if that is not possible, only then use the mut'a marriage.
The temporary nature of mut'a can also be seen from the following saying of the Imams: Once 'Ali bin Yaqtin, a prominent Shi'ah who held a high post in ' Abbasid government, came to Imam 'Ali ar-Riza to ask about mut'a. The Imam said, "What have you to do with it because Allah has made you free from its need." (Wasa'il, vol. 14, p. 449) He has also said, "It is permitted and absolutely allowed for the one whom Allah has not provided with the means of permanent marriage so that he may be chaste by performing mut'a. (Wasa'il, vol. 14, p. 449-450)