1. The Lawful and Unlawful in Regards
Plural Marriage, Fornicators and Temporary Marriage

2. When we speak of fornicators here we are referring to Prostitutes
It is reported that Marthad ibn Abu Marthad asked the Prophet's permission to marry a prostitute named 'Anaq with whom he had relations during the pre-Islamic period. Allah then sent down the following verse of the Quran in response:
“The fornicator shall not marry anyone except a fornicatress or an idolatress, and the fornicatress shall not marry anyone but a fornicator or an idolater, and that (marrying them) is haram for the Believers. “ (24:3)
The Prophet then recited this verse to Marthad and said, "Do not marry her."
(This story is reported by Abu Daoud, al- Nisai, and al-Tirmidhi.)

3. Allah has permitted Muslims to marry chaste believing women or chaste women of the People of the Book.
Similarly, He has made marriage lawful to men on the condition that they seek it "in honest wedlock, not in lust." (4:24).
In other words, one can not marry for lust and if someone does not accept this command from the Book of Allah, nor considers it binding, he is a Unbeliever
For those people who have relations outside of marriage, they are considered fornicators and the perscribed punishment for the unmarried fornicator and his partner is:
“ Flog the woman and the man guilty of fornication each with a hundred stripes....” (24:2)

4. Women and men who have relations outside of marriage are low grade and Allah has ordained that marriage be a source of affection and mercy between the spouses
How then could a vile woman or man be the object of love? As vileness and virtue are antithetical to each other both by nature and by considerations of morality, there cannot even be a sympathy, much less love and affection, between the two.
Indeed, Allah has spoken truly in His saying,
“Vile women are for vile men, and vile men are for vile women; virtuous women are for virtuous men and virtuous men are for virtuous women.”
(24:26)

5. What about Mutah or Temporary Marriage?
Marriage in Islam is a strong bond and binding contract, based on the intention of both partners to live together permanently in order to attain, as individuals, the benefit of the repose, affection, and mercy which are mentioned in the Qur'an, as well as to attain the social goal of the reproduction and perpetuation of the human species:
“And Allah has made for our spouses of your own nature, and from your spouses has made for you sons and grandsons....” (16:72)
Temporary marriage (known in Arabic as mut'ah), which is contracted by the two parties to last for a specified period of time in exchange for a specified sum of money, violates the purpose of marriage.
The Prophet made this practice Haram or Forbidden

6. Temporary marriage used to be practiced by the Arabs during wartimes
Temporary marriage used to be practiced by the Arabs during wartimes. Imam Bukhari narrates that Ibn Masud said:
“We were on an expedition with the Prophet and did not have our wives with us, so we asked Allah's Messenger, 'Should we not castrate ourselves? (The reason for this request was the desire to maintain their purity of mind and body, which was in danger of being affected by their un met needs. He forbade us to do so but permitted us to contract marriage with a woman up to a specified date, giving her a garment as a dower (mahr)
(Reported by al-Bukhari and Muslim.)
Later this was outlawed by Allah and his Prophet. Narrated 'Ali:
“I said to Ibn 'Abbas, "During the battle of Khaibar the Prophet forbade (Nikah) Al-Mut'a and the eating of donkey's meat." (Bukhari)

7. Also before Islam it was the practice of men of all religions to have more than one wife. Islam came about and set a limit for men in this:
With regard to the restriction, it limited to four the maximum number of wives a man might have. When Ghailan al- Thaqafi accepted Islam, he had ten wives. The Prophet told him:
"Choose four of them and divorce the rest,''
(Ahmad, al-Tirmidhi, Ibn Majah)
Many people ask well how is it that the Prophet had eleven wives.The case of the Prophet was exempted from this by Allah for the sake of da'wah during his lifetime and because of the need of the Muslim ummah after his death.

8. Not only did islam limit the amount of wives allowed a man but also added a condition:
The condition which Islam lays down for permitting a man to have more than one wife is confidence on his part that he will be able to deal equitably with his wives in the matter of food, drink, housing, clothing and expenses, as well as in the division of his time between them. Anyone who lacks the assurance that he will be able to fulfill all these obligations with justice and equality is prohibited by Allah from marrying more than one woman, for Allah says:
“...But if you fear that you will not be able to do justice (among them), then (marry) only one....” (4:3)
And the Prophet said,
“Anyone who has two wives and does not treat them equally will come on the Day of Resurrection dragging one part of his body
( Ibn Hibban and al-Hakim.)