The term encompasses polygyny (the practice of one husband having multiple wives); polyandry (the practice of one woman having multiple husbands); and group marriage (the practice of two or more men and two or more women sharing husbands and wives, also known by the slang term swinging). It can also involve three or more people of the same sex in such an arrangement.
In modern times, opponents of same-sex "marriage" worry that if legalized it would be a step down the slippery slope to polygamy. Recent events to support this claim includes the Netherlands in 2005 four years after introducing same-sex 'marriage' gave three-people relationships legal recognition through a "cohabitation agreement". Stanley Kurtz wrote:
- Among the likeliest effects of gay marriage is to take us down a slippery slope to legalized polygamy and "polyamory" (group marriage). Marriage will be transformed into a variety of relationship contracts, linking two, three, or more individuals (however weakly and temporarily) in every conceivable combination of male and female.
Polygamy, in ancient times, was widespread, but has become less common over time. The most common form of polygamy is polygyny. Polygynous marriages are found in the Old Testament, especially among the early Hebrew kings. The first recorded polygamist was Lamech (Genesis 4:19), in the seventh generation from Adam. The book of Deuteronomy gives an inheritance rule for polygynous families. The culmination was Solomon with 700 wives and 300 concubines; however, the fact that they were from the pagan nations led him down the path of corruption. There was also a practice of giving a maid servant to a husband to sire children in the period of the patriarchs; Scripture refers to such a maid servant variously as a wife and as a concubine. God, speaking through the prophet Nathan, addressed David's polygamy without expressing disapproval:
And Nathan said to David, Thou [art] the man. Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul; And I gave thee thy master's house, and thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if [that had been] too little, I would moreover have given unto thee such and such things. 2Samuel 12:7-8 (KJV)
There are examples of polygamy in the New Testament and it is stated that a deacon is to be the husband of one wife. That the church leaders saw fit to suspend the practice for such a small group, but allow it for everyone else speaks volumes. (in some Christian denominations today, that has been taken to mean that divorced men and in some cases even widowed men can not be deacons if they remarry.)
Ashkenazi Jews banned polygyny around A.D. 1000. Sephardi Jews continued to practice it in Islamic lands for some time after that. Yemeni and Ethiopian Jews practice it to this day. Israel forbids polygyny, with the exception that a man with multiple wives who moves to Israel may remain married to his existing wives but not marry additional ones. However, this is due at least in part to an Israeli Supreme Court ruling that requires recognition of all marriages performed abroad, whether they would be legal in Israel or not. (This also applies to same-sex "marriage").
In the modern world polygyny is practiced to some extent in China, among certain sects of Mormons (the mainstream Mormon Church outlawed the practice in 1890) and in parts of Sub-Saharan Africa.
"Group marriage" is likely the least common form of polygamy, but it is practiced. In fact a number of Americans purport to practice just this sort of living arrangement, even though it is not legally recognized as marriage by the state.
The Bible clearly states that marriage, as recognized by God, involves only one man and only one woman; therefore, any other type of "marriage" (whether or not formally legalized) is prohibited, notwithstanding some liberal denominations holding to the contrary.
- Beyond Gay Marriage, Stanley Kurtz
- Deuteronomy 21:15-17
- I Kings 11:1-3
- Genesis 30:4
- Genesis 35:22
- I Timothy 3:2
- Titus 1:6
- Judaism 101: Marriage
- "... Marry women of your choice, Two or three or four ..." Koran 4:3-4
- Polygyny in Europe and China
- Lang and Smart. “Migration and the Second Wife in South China: Toward cross-border polygyny” The International migration review 2002, vol. 36, no2, pp. 546-569 
- News story on Fundamentalist Church of Latter Day Saints
- LDS official position
- Brown, Judith E. “Polygyny and Family Planning in sub-Saharan Africa” Studies in Family Planning, Vol. 12, No. 8/9 (Aug. - Sep., 1981), pp. 322-326. Abstract. 
- List of references
- Polyandry among the Inuit 
- Kurtz, Stanley, “Rick Santorum Was Right Meet the future of marriage in America.” New Republic Online 3/23/05