In an earlier post, Married Saints – Why so few?, I addressed the question of why there are so few married saints canonized as married saints, that is, in view of the life they lived as married persons. In the comment thread to that post, I was asked why so many of the married persons who have been canonized lived in continence, that is, without having sexual intercourse with their spouse for a significant portion of their life as married persons.
Again, there are several possible answers, grouped according to the general manner they explain the connection between this continence and canonization.
There is a positive correlation from continence to charity (continence contributes to charity, or is thought to do so)
(1a) Such continence is in fact extremely helpful, indeed practically necessary in order to attain the heroic virtue to which canonization attests.
(1b) Such continence was thought to be necessary in order to attain the perfection of charity.
Amongst all relationships, conjugal affection engrosses men's hearts more than another other, so that our first parent said: "A man leaves father and mother, and clings to his wife" (Gen. 2:24). Hence, they who are aiming at perfection, must, above all things, avoid the bond of marriage.
The second way to perfection, by which a man may be more free to devote himself to God, and to cling more perfectly to him, is the observance of perpetual chastity… The way of continence is most necessary for attaining perfection… Abraham had so great spiritual perfection in virtue, that his spirit did not fall short of perfect love for God on account either of temporal possessions or of married life. But if another man who does not have the same spiritual virtue, strives to attain perfection, while retaining riches and entering into marriage, his error in presuming to treat Our Lord's words as of small account will soon be demonstrated. (St. Thomas Aquinas, On The Perfection of the Spiritual Life; this quotation, from a saint and universal doctor of the Church, is intended as support for 1a and 1b.)
There is a positive correlation from continence to canonization
(2) The holiness of married saints who practiced such continence is more evident than the holiness of others.
One reason for this, as I mentioned in the previous post, is that holiness always involves following the spirit of the evangelical counsels (poverty, chastity, obedience); and other things being equal, someone's following the spirit of the counsels is more evident when it is incarnated in the literal following of the counsels.
There is a positive correlation from holiness to continence
(3) Those who are well advanced in charity and the other virtues are disposed and desirous of practicing such continence. (This may follow to some extent of itself, and to so extent due to 1b.)
Fulton Sheen, in his work Three to Get Married, suggests something along these lines:
All love is a flight towards immortality. There is a suggestion of Divine Love in every form of erotic love, as the lake reflects the moon…. Sex is only the self-starter on the motor of the family…. The begetting of children enlarges the field of service and loving sacrifice for the sake of the family. In a well-regulated moral heart, as time goes on, the erotic love diminishes and the religious love increases. In marriages that are truly Christian, the love of God increases through the years, not in the sense that husband and wife love one another less, but that they love God more. Love passes from an affection for outer appearances to those inner depths of personality which embody the Divine spirit. There are few things more beautiful in life than to see that deep passion of man for woman, which begot children, transfigured into that deeper passion for the Spirit of God. It sometimes happens in a Christian marriage that when one of the partners dies, there is no taking of another spouse, lest there be the descent to lower realms from that higher love, from the Agape to the Eros.
As before, so here I suggest the answer is, in varying degrees: all of the above. Continence in its various forms (the periodic continence practiced in NFP, continence during times of more intensive prayer (e.g., Lent) mentioned by St. Paul, or continence after the children-bearing time) is a valuable means to growth in the gift of oneself implied in charity; it was considered to be a valuable, practically necessary means; it manifests virtue; and it often flows naturally from charity.
A few points to be made pertinent to the remarks of the commentator in the previous post
(a) A spiritual director might rightly refrain from taking any initiative in advising a particular couple to such continence for a long period, and might caution them if they are desirous of practicing it for a long period. That does not mean, however, that he would or should strongly disallow or strongly advise against it.
(b) There have definitely been various developments in the Church's understanding of virginity and marriage. It seems quite true to say that in praising virginity and continence, marital relationships were not infrequently excessively devalued. There are various reasons for this, one of which is that in general there was a greater concern to safeguard the special value of virginity than of marriage. Hence, if it was difficult to avoid either failing to properly appreciate virginity or failing to properly appreciate marriage, as it was and is difficult for people to properly appreciate both, they preferred to fail to properly appreciate marriage rather than to fail to appreciate virginity, with the natural consequence that in many cases they did fail to properly appreciate marriage.
(c) To affirm a greater possibility of love in giving sex up for the sake of a greater good, as in the case of celibacy or continence, does not imply that sex is bad or even hinders any particular degree of holiness, anymore than the affirmation that "there is no greater love than this, that a man lay down his life for his friends" means that life is bad, or that living is an obstacle to becoming holy.