Someone recently asked about God's definition of marriage.

Here are relevant portions of an article on Marriage from the Christian Cyclopedia (which can be accessed at http://www.lcms.org/ca/www/cyclopedia/02/ ):

BIBLICAL HISTORY: In the Old Testament (OT) it is implied that marriage is an expression of the will of God (e.g., Gn 2:18–24). OT prophets abundantly defend sexual purity and domestic virtue. Blessings of marriage and evils of impurity are often used figuratively (e.g., Rv 21:2; Ex 34:15–16).

Monogamy was considered ideal in the OT (Gn 2:24; Pr 31:10–31), but there is no OT prohibition of polygamy, which persisted well into the Christian era and is not forbidden in the New Testament (NT) but conflicts with the NT ideal of monógamy (1 Ti 3:2, 12; Tts 1:6; cf. Jn 4:18).

DEFINITION AND PRINCIPLES: A.L. Graebner defined the state of marriage, or wedlock, as “the joint status of one man and one woman, superinduced and sustained by their mutual consent to be and remain to each other husband and wife in a lifelong union for legitimate sexual intercourse, the procreation of children, and cohabitation for mutual care and assistance.”

On basis of Gn 1:27–28; 2:21–24; Ro 1:26–27 Christians oppose homosexual marriage.

According to Scripture, marriage has a 3-fold purpose: companionship and mutual help; procreation; and, since the fall of man into sin, avoidance of fornication (Gn 1:28; 2:18–24; 1 Co 7:2).

Marriage is intended by God to be lifelong (Mt 19:6; Mk 10:9; Ro 7:2; 1 Co 7:39).