The main question, among several questions, about forms of marriage or no-marriage is not morality, but what is the best form of child rearing for modern societies. There is every variety to choose from - monogamy, polygamy, state care, village care, foster mothers, boarding schools.
Zoology showcases every imaginable form of male-female care of offspring - from babies born immediately into the wide world, self-sufficient, needing no care at all, to life-partnerships for rearing a series of infants that need nurture as well as instincts to know how to survive.
Most human cultures have norms for long-term monogamous heterosexual adult partnerships as giving their young the most security and opportunities for care and training throughout their long childhoods. Cultures have varied in the degree to which sexual relationships have been tolerated outside marriage, especially for women. People today may not realise the differences since there has been more effective contraception, state support for children out of wedlock, better treatments for venereal diseases, safer communities, and a commercially-promoted sexualised culture.
But most Western children aren’t growing up within permanent monogamous heterosexual marriages. Leaving aside whether this sort of marriage is divinely ordained, is it ordained by nature? Humans are products of their cultures as much as of their biology.
Polygamy (usually multiple wives) is useful in societies where there are high death rates for young males and, where widows are left destitute. In parts of the world, extra wives and their extra children are valued as labour, and women are tradable goods. Some recent American polygamous relationships are sustained by child-welfare payments. The British furore against the idea of sanctioning polygamy has been emotionally fuelled by the thought that taxpayers could be funding men to accumulate offspring.
Polygamy can mean that many young men cannot marry. Powerful men may accumulate women as a sign of status. Harems can suffer from jealousy and struggles to obtain preferences for children, and the old wife losing position to the younger. Cast-off wives may have no way to survive. The man is almost inevitably the dominant partner, so the idea can seem attractive. A high proportion of current top Google references on polygamy are arguing for the practice.
Polyandry, with one woman married to several men, usually brothers, is rare and has commonly served to avoid division of small landholdings. Polyandry has recently been suggested where marriageable females are now in short supply through practices of killing girl fetuses and infants. A better solution might be for parents to realise that daughters, rather than sons, are now more likely to be caring of them in their old age, and accordingly to revalue the gender of their children.
In addition to the legal institution of lifetime heterosexual monogamy Western societies have, in order of prominence, single mothers, de facto relationships, serial monogamy, same-sex parents, and instability of all relationships. The overall evidence indicates that children are likely to flourish most when brought up in long-term stable and loving relationships, whatever these may be.
But it is better that children grow up with separated parents than experiencing continual hatred within the family - but better still if both parents can manage to behave courteously during the formative years of children and young teens.
But marriage is not only for sake of the children, regardless of some clerical claims. Church marriage services themselves, such as the traditional Anglican Book of Common Prayer have set out three reasons why God ordained marriage. Two are well-known - children and assured sexual relationships (“With my body I thee worship” or, at least, “It is better to marry than to burn”.) The third reason is often forgotten, “for the mutual help and comfort, one of the other”.
In the old days, without state support in times of trouble, this symbiosis was taken seriously. Young lovers sang ballads about girls who could “bake and brew” and young women put “a good provider” high on their lads’ attractions. Marriages were to provide support in times of trouble, rather than the opposite, to be more at risk of break-up.
The ideal of “help and comfort” in marriage meant through thick and thin, in sickness and in health, secure in trust and loyalty, and within the supporting structures of society.