Surveying world religions and cultures, marriage can be described as a life-long agreement between two or more people typically involving three commitments: some degree of cohabitation, some degree of exclusivity in sexuality, and some degree of mutual effort in the rearing children.
Given the importance of marriage for societal stability, all religions legitimated marriage by linking it to a transcendent order: some religions say marriage is divinely instituted, others say marriage is a metaphor for divinity itself, other religions say marriage produces an everlasting metaphysical bond between partners.
Religions also legislate marriage in order to legislate sex. But most ancient religious prohibitions against certain sexual acts were really concerned with matters other than the sexual acts themselves. Consider:
Prohibitions against adultery were really concerned with the desire of men to have assurances of paternity (i.e., who their children were). In antiquity, a prohibition against adultery was addressed to women, not men. Married men could have multiple sex partners (multiple wives or concubines) as long as these women only had this one man as their sex partner. Since the men wanted to know who their children were, it was necessary that women have only one man as a sex partner. And so, there emerged a religious prohibition against female adultery. (Later, the prohibition came to apply to men too; although some religions still permit men to have multiple wives).
Prohibitions against fornication (sex without marriage) were really concerned about unwanted children. If children emerged from these sexual encounters and the sex partners were not committed together to raise those children, then the children became unwanted and burdensome to a community. And so, there emerged a religious prohibition against fornication and thereby a legitimating of marriage.
Prohibitions against homosexuality were really concerned with fertility and childbirth. In antiquity, every tribe that desired to establish itself in an enduring way had to ensure fertility, multiple births, and multiple children. In some religions, sexuality that was not open to procreation was prohibited and deemed intrinsically evil. The ancients especially targeted male masturbation, male withdrawal of the penis from the vagina to ejaculate, and male homosexuality. Since every ejaculation had to occur within a vagina so that conception might occur, wasting the seed (in any manner) was condemned in the strongest language. And so, there emerged religious prohibitions against male masturbation, and withdrawal to ejaculate, and male homosexuality.
Questions: In an age of DNA testing is paternity still a neurotic concern of husbands? In an age of reliable birth control does sex between unmarried partners always end in unwanted pregnancy? In an age of population boom and limited resources must every sex act produce a child?
If the conditions for these prohibitions have vanished over time, why do religious prohibitions against adultery, fornication, and homosexuality persist?
Why is marriage the only conceivable option?