All the laws of truth and right flow from celestial beginnings, or from the order of life of the celestial man. For the whole heaven is a celestial man because the Lord alone is a celestial man, and as He is the all in all of heaven and the celestial man, they are thence called celestial.
As every law of truth and right descends from celestial beginnings, or from the order of life of the celestial man, so in an especial manner does the law of marriages.
It is the celestial (or heavenly) marriage from and according to which all marriages on earth must be derived; and this marriage is such that there is one Lord and one heaven, or one church whose head is the Lord. The law of marriages thence derived is that there shall be one husband and one wife, and when this is the case they represent the celestial marriage, and are an exemplar of the celestial man.
This law was not only revealed to the men of the Most Ancient Church, but was also inscribed on their internal man, wherefore at that time a man had but one wife, and they constituted one house. But when their posterity ceased to be internal men, and became external, they married a plurality of wives. Because the men of the Most Ancient Church in their marriages represented the celestial marriage, conjugial love was to them a kind of heaven and heavenly happiness, but when the Church declined they had no longer any perception of happiness in conjugial love, but in pleasure from a number, which is a delight of the external man. This is called by the Lord “hardness of heart” on account of which they were permitted by Moses to marry a plurality of wives, as the Lord Himself teaches:
For the hardness of your heart Moses wrote you this precept, but from the beginning of the creation God made them male and female. For this cause shall a man leave his father and mother, and shall cleave unto his wife, and they twain shall be one flesh; wherefore they are no more twain but one flesh; what therefore God hath joined together let not man put asunder (Mark 10:5-9).