One of the finest discussions of this question is given by Germain Grisez in his book, Living a Christian Life, Vol. 2 of his The Way of the Lord Jesus, pp. 721-737. In Love and Responsibility (1960) Karol Wojtyla has some helpful things to say, particularly in the final chapter on sexology. I will first summarize material from Grisez, then from Wojtyla, and finally offer some practical suggestions.
1. If marital troubles arise as a result of moral failures
Very often marital problems arise from different moral failures, usually by both husband and wife. At other times nonmoral factors can cause them. Since God’s grace and human persons cooperating with his grace can overcome all serious sins, the moral failings troubling a marriage can be overcome if the couple uses the means necessary to do so. At times venial sins can cause difficulties which a couple may not be able to overcome entirely (e.g., using profane language, getting impatient), but they must learn to live with such failings on each other’s part. And if we say we are without venial sin we are liars. Usually, difficulties begin to trouble a marriage when one or the other spouse (or both) is doing something he or she should not do or is failing to do what he or she ought to do; naturally this does not please the other and a conflict brews. Such difficulties almost always involve a moral conflict rooted in some sin of at least one of the spouses and frequently in ongoing, more or less serious, sins of both. Among moral failings leading to or aggravating marital troubles, the failure to fulfill one’s duty to communicate, to express feelings and thoughts carefully and gently, to listen attentively and sympathetically is of special importance because failure to communicate makes it very hard if not impossible for the couple to cooperate in developing their special friendship and in dealing with difficulties that arise. There are various ways of improving communication, among them learning how men and women differ in the way they think, use language, etc. It seems to me that men in particular have a problem in “listening” to their wives. A friend once told me one ought to “listen with one’s eyes as well as ears.” Doing so can work wonders.
To overcome difficulties rooted in moral failures, husbands and wives must be ready to admit these failures, repent of them, and stop doing them. The Church teaches (see Council of Trent, decree of justification) that God gives sufficient grace to enable Christians willing to repent and cooperate with grace to avoid serious sin. Couples should repent, forgive, and admonish each other, and do the latter gently. They absolutely must use the supernatural means necessary and available to overcome their own moral failures: prayer, personal and as a couple, the sacrament of penance, Mass and reception of communion as often as possible. They should pray for each other. If they do this, and one spouse or the other thinks that the other has not repented some serious fault causing trouble, he or she should admonish the apparently sinful spouse in a way appropriate to a married couple and in a way likely to help rather than aggravate. The admonishing spouse should appeal to the common good of marriage, which consists in their unique communion of persons imaging on earth and making present here and now the communion of the Divine Persons. This good ultimately results in their sanctification, and shows how the spouse’s behavior harms this good in a specific way. At the same time, the admonishing spouse should reaffirm his or her love and make it clear that no grudge is held. Spouses must void: (1) using admonition as a way of striking back; (2) appealing to selfish interests of their own; (3) becoming so discouraged that they end up condemning the other as hopeless. (See Grisez, pp. 721-723).
2. If marital troubles are rooted in nonmoral sources
Sometime marital discord stems not from moral failure but from physical or psychological factors that lead spouses to behave badly (e.g., ulcers, neuroses of various kinds). As a result of these spouses may become easily upset and irritable and spouses affected by them commit venial sins of various kinds: verbal abuse, sulkiness, refusal to cooperate in small matters. Although such sins are often neither deliberate nor serious in themselves, the other spouse recognizes them as bad and is likely to react sinfully and conflicts arise; these can seriously disrupt marital harmony.
At other times marital discord arises from other nonmoral evils, e.g., a lack of communication skills, ignorance of key differences between men and women, sexual clumsiness and impatience with other partner in sexual relations.
Frequently when marital troubles arise from these sources husbands and wives need the help of professionals (medical doctors for physical problems, psychologists/psychiatrists for problems caused by neuroses etc., good therapists for sexual problems). (See Grisez, pp. 723-725).
In the final chapter, “Sexology and Marriage,” of Love and Responsibility, in considering “marriage and marital intercourse,” the future Pope John Paul stresses that “certain findings of sexology that enable us to understand more fully how sexual stimuli affect men and women (boys and girls) differently, how boys and girls differ in their sexual awakening. Such information can be of value in understanding better the complex of somatic and physiological factors conditioning the sensual reactions in which the sexual urge manifests itself” (pp. 268-269).
In discussing marriage and marital relations he is principally concerned with making males aware of the very different way in which sexual excitement reaches its climax in females than in males. He argues that "from the point of view of another person, from the altruistic standpoint, it is necessary to insist that intercourse must not serve merely as a means of allowing sexual excitement to reach its climax in one of the partners, i.e., the man alone, but that climax must be reached in harmony, not at the expense of one partner, but with both partners fully involved" (p. 272). In short, husbands ought to learn how to please their wives by becoming familiar with the findings of sexology in this matter. "Non-observance of these teachings of sexology in the marital relationship,” he says, “is contrary to the good of the other partner to the marriage and the durability and cohesion of the marriage itself" (p. 273). If insufficient heed is paid to such truths, “the wife, who will not be fully involved, may begin to have a hostile attitude toward sex, become frigid in some way, and even result in psychological and physiological damage to the woman” (p. 273). He thinks that it is inappropriate for the wife to "sham orgasm," because this conceals the problem and can at best be a palliative. He pushes for true personal education in the matter and distinguishes between a "culture of marital relations" and concern for mere technique–the "how to" manual approach (pp. 274-275). What is most needed is true love.
Some practical suggestions
To avoid and overcome marital problems I think that the following suggestions are helpful:
1. If one is ever inclined to think that he or she would be better off if he or she had not married this particular person, one ought to regard this thought as a proximate occasion of sin and immediately put this idea out of one’s head.
2. If one is becoming unhappy with one’s spouse, one should focus attention on the reasons why one wanted to marry this person to begin with.
3. Every day, perhaps best as you are both going to sleep at night, tell your spouse that you love her/him and are happy that you are married to him/her.
4. Do not go to bed angry with one another; rather beg forgiveness for anything you have done or failed to do that causes unhappiness.
5. Once you have forgiven a spouse for a serious moral failure, don’t remind him or her of it every time you get irritated. Forgive and forget!