Proverbs 5:18

Ye wives, be subject to your own husbands, and have them in esteem, and serve them with fear and love, as holy Sarah honoured Abraham. For she could not endure to call him by his name, but called him lord, when she said, "My lord is old." In like manner, ye husbands, love your own wives as your own members, as partners in life, and fellow-helpers for the procreation of children. For says He, "Rejoice with the wife of thy youth. Let her conversation be to thee as a loving hind, and a pleasant foal; let her alone guide thee, and be with thee at all times: for if thou beest every way encompassed with her friendship, thou wilt be happy in her society." Love them therefore as your own members, as your very bodies; for so it is written, "The Lord has testified between thee and between the wife of thy youth; and she is thy partner, and another has not made her: and she is the remains of thy spirit;" and, "Take heed to your spirit, and do not forsake the wife of thy youth."

"Let your fountain be blessed," etc. Maintain such temperance with the woman you married in your youth, even in old age, that by faithful chastity you may rightly gain a blessing in your offspring. "A loving doe, a graceful fawn." Let her always be your beloved spouse, who, like a doe that avoids serpents, shuns harlots and drives them away from her home. Let a son be born from her, and he himself be a very strict lover of chastity.

The same principle (by the way) applies to the difficult problem of wives. Variability is one of the virtues of a woman. It avoids the crude requirement of polygamy. So long as you have one good wife you are sure to have a spiritual harem.