There is only one issue I address up front in that Tolkien makes a passing reference to Brigham Young. It is a fairly positive "humorous" comment, yet for LDS readers and maybe others it tends to distract from the bigger theme unless we just acknowledge it and state that neither I nor the Church of Jesus Christ of Latter-day Saints advocate for plural marriage (at least since 1890 - well before my time).
With that out of the way, here is a portion of Tolkien's letter to his son:
. . . .[Women] are instinctively, when uncorrupt, monogamous. Men are not. . . . . No good pretending. Men just ain't, not by their animal nature. Monogamy (although it has long been fundamental to our inherited ideas) is for us men a piece of 'revealed' ethic, according to faith and not to the flesh. Each of us could healthily beget, in our 30 odd years of full manhood, a few hundred children, and enjoy the process. Brigham Young (I believe) was a healthy and happy man. It is a fallen world, and there is no consonance between our bodies, minds, and souls.
However, the essence of a fallen world is that the best cannot be attained by free enjoyment, or by what is called 'self-realization' (usually a nice name for self-indulgence, wholly inimical to the realization of other selves); but by denial, by suffering. Faithfulness in Christian marriage entails that: great mortification. For a Christian man there is no escape. Marriage may help to sanctify & direct to its proper object his sexual desires; its grace may help him in the struggle; but the struggle remains. It will not satisfy him - as hunger may be kept off by regular meals. It will offer as many difficulties to the purity proper in that state, as it provides easements. No man, however truly he loved his betrothed and bride as a young man, has lived faithful to her as a wife in mind and body without deliberate conscious exercise of the will, without self-denial. Too few are told that - even those brought up 'in the Church.' Those outside seem seldom to have heard it. When the glamour wears off, or merely works a bit thin, they think they have made a mistake, and that the real soul-mate is still to find. The real soul-mate too often proves to be the next sexually attractive person that comes along. Someone whom they might indeed very profitably have married, if only --. Hence divorce, to provide the 'if only.' And of course they are as a rule quite right: they did make a mistake. Only a very wise man at the end of his life could make a sound judgement concerning whom, amongst the total possible chances, he ought most profitably to have married! Nearly all marriages, even happy ones, are mistakes: in the sense that almost certainly (in a more perfect world, or even with a little more care in this very imperfect one) both partners might have found more suitable mates. But the 'real soul-mate' is the one you are actually married to. You really do very little choosing: life and circumstance do most of it (though if there is a God these must be his instruments, or His appearances). It is notorious that in fact happy marriages are more common where the 'choosing' by the young persons is even more limited, by parental or family authority, as long as there is a social ethic of plain unromantic responsibility and conjugal fidelity. But even in countries where the romantic tradition has so far affected social arrangements as to make people believe that the choosing of a mate is solely the concern of the young, only the rarest good fortune brings together the man and woman who are really as it were 'destined' for one another, and capable of a very great and splendid love. The idea still dazzles us, catches us by the throat: poems and stories in multitudes have been written on the theme, more, probably, than the total of such loves in real life (yet the greatest of these tales do not tell of the happy marriage of such great lovers, but of their tragic separation; as if even in this sphere the truly great and splendid in this fallen world is more nearly achieved by 'failure' and suffering). In such great inevitable love, often love at first sight, we catch a vision, I suppose, of marriage as it should have been in an unfallen world. In this fallen world we have as our only guides, prudence, wisdom (rare in youth, too late in age), a clean heart, and fidelity of will. . . . .
J.R.R. Tolkien, From a letter to Michael Tolkien, 6-8 March, 1941 in The Letters of J.R.R. Tolkien, selected and edited by Humphrey Carpenter (Houghton Mifflin Co., Boston 1981) pp. 51-52 (emphasis in the original).So, do you see that I am still a social conservative in my very own way?
The long letter goes on to explain the history of Tolkien's courtship (including some family conflicts not to mention the intervention of WWI) and indicates his great love for his spouse. What son would not appreciate a letter like that? I hope my children understand this. I have attempted to address these issues in ways difficult to assess. I do hope they see in practice the great love their parents have by strong commitment to each other - the fidelity of will Tolkien talked about.
Tolkien was obviously a romantic. His books should convince you of that. And if you know his story of Beren and Luthien, his gravestone will make even more sense in that regard.