Marriage
Marriage is a form of interpersonal relationship; a kind of social contract. Most often, it takes the form of a union between a man and a woman. In a few times and places, other forms existed, such as polygyny (one man and more than one woman), polyandry (one woman and more than one man), and group marriage (any number of men, any number of women).
In the civilized Western world marriage as an institution between one man and one woman has been a cornerstone of community for many hundreds of years. Recently there has been a push by liberals for "same-sex marriage". A large portion of the reason for this push is because of the benefits married people gain in the United States, especially in regard to property rights. Same-sex marriage has no basis in scripture. In United States law, marriage is defined by each state and same-sex marriage is allowed only in Massachusetts. It might be worthwhile to note that interracial marriage was illegal in most states during the 19th century[1].
Bible
The Old Testament clearly allows polygyny. (Gen. 4:19, Gen. 16:1-4, Gen. 26:34, Gen. 32:32, Exodus 21:10, Dt. 21:15, Judges 8:30, I Sam 1:1-2, II Sam 12:7-8, I Kings 11:2-3, I Chron. 4:5, II Chron. 11:21, II Chron. 13:21, II Chron 24:3.) This is less common, but never outlawed, in the New Testament.
The Old Testament allows for the purchase of wives (Gen. 29:18, I Sam. 18:25-27).
The clearest description of marriage comes from I Corintians 7:1-16:
Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman. Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband. The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife. Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
But I speak this by permission, and not of commandment. For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that.
I say therefore to the unmarried and widows, It is good for them if they abide even as I. But if they cannot contain, let them marry: for it is better to marry than to burn.
And unto the married I command, yet not I, but the Lord, Let not the wife depart from her husband: But and if she depart, let her remain unmarried, or be reconciled to her husband: and let not the husband put away his wife.
But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away. And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him. For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy. But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace. For what knowest thou, O wife, whether thou shalt save thy husband? or how knowest thou, O man, whether thou shalt save thy wife?
Christianity
In most branches of Christianity, marriage is a life-long religious union between a man and a woman.
Mormonism
Historically, Mormonism has practiced polygyny. It stopped being a part of mainstream Mormonism in 1890, but some fringe groups still practice it.
Civil marriage
Civil marriage is a legal contract. In the state constitutions of almost all states, this contract must be between one man and one woman. It gives each member certain rights and obligations. Most commonly, these include power of attorney, inheritance rights, and rights in relation to any children they may have.
