Polygamy is an inclusive term for several variant forms of marriage. It encompasses polygyny, the practice of having multiple wives, polyandry, the practice of having multiple husbands, and group marriage, the practice of having multiple husbands and wives. Any marriage of three or more people is polygamous.
In modern times, opponents of "gay marriage" worry that if legalized it would be a step down the slippery slope to polygamy. 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. 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.
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 .
Polyandry is and has been less common around the world, but it has been practiced. Polyandry is part of traditional Tibetan culture and is practiced by some Native American tribes. The Bible prohibits polyandry.
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..
- 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 
- "Polygamy," from A True Church 
- Kurtz, Stanley, “Rick Santorum Was Right Meet the future of marriage in America.” New Republic Online 3/23/05