I've been in a conversation with an acquaintance about the Sacrament of Matrimony. This is part of the conversation that I've been having with him. While I do know that it isn't complete, it is an important part of understanding why the Church holds the Sacrament of Matrimony and the parts of it including sex to a higher standard. Please read on:
The Church has for all time understood that there are situations in which fertility issues exist between couples. For a good many years, the Church forbade couples who were infertile from marrying, UNLESS there was a dispensation. Over time, that dispensation became more and more liberalized, because the Church, in her wisdom understood the importance of the Sacrament of Matrimony and realized there was more to being married than just having children. That isn't to say though that having children isn't part of the proper nature of the Sacrament.
This dispensation still exists and it still must be sought out if a couple wants to marry, but have fertility issues. It is however, one of the most liberal dispensations given and rightly so. However, there is a trade off. I'm not going to go into frequency or infrequency of the sex act, but I will say this. There is more to the Sacrament of Matrimony than simply having children.
The Sacrament of Matrimony also includes a sense of complimentarity between a man and a woman. The Sacrament draws out the love of a man and a woman ordered to the divine love of God. That is the first end of marriage and that is why people get married. To have a child is the physical outpouring of that love. This is one of the reasons that having children is so important. It is sort of a manifestation of the Sacrament. If a Sacrament is the outward sign instituted by Christ to bring about Grace, then having a child is the manifestation of it in an informal and symbolic way. It is not the only way, but it is the most visible way.
That being said, if one cannot have children, then one must understand that being married in the Church requires a dispensation. Why? Because they must be open to the ability to have children. If one or both have fertility issues, then one must ask permission (in a very real sense) to do something which is outside the scope and meaning of the Sacrament. It is understandable.
Let's assume for a moment that there is a married couple who has had a "normal" marriage wherein there were not fertility issues. As time passes, the development of their love grows. The husband and wife move from child bearing into child rearing and eventually into retirement. The sex act also changes. As they decide that they've accomplished all that God has asked of them (notice the difference in attitude), the love shifts from erotic love to a more complete love. This does not necessarily entail the sex act. But also thinking back, there are times in the marriage where it is not appropriate to engage the sex act. I'm not talking about a fleeting weekend or day. I'm talking about an extended period of time. This is when continence plays a part.
The Church teaches that part of the responsibility of the Sacrament is to practice continence, when necessary. The key is when necessary. For some that is never. For some, that is part of the time, and for some that is frequently. And finally, for some that is total. The Church doesn't tell a couple when that time is, but the Church does say that when faced with having to use contraception or be put in a position of needing to procure and abortion, it is best to be continent. The rhythm method can be of use, as long as it is not used as birth control. If it is used as birth control and not a way to time fertility, then it is just as illicit as using a condom or the pill. An important part of the rhythm method which is not spoken of is continence. It is not always appropriate to engage the sex act when following this method. There is a great amount of responsibility needed.
Returning to the idea of continence, as a couple grows older, the need for sex lessens. The type of love transforms from one of needing sex to complete the couple to finding ultimate fulfillment in the love the one person has for the other. This is the true end of the Sacrament.
Moving to the couple who has fertility issues, they are asked to find this ultimate fulfillment faster. For living a chaste life within marriage may mean that they don't have the opportunity to engage the sex act as often or for as long. Chasitity in marriage is much more complex and exacting than it is for the single person, because the number of people who are expected to find complemenatarity has doubled.
What I'm getting at is that in today's world, people have forgotten what chastity is. Having sex for the sake of having sex, even in marriage is not being chaste. While the sex act has two parts to it, unitive and procreative, if a married couple closes one of those areas, then they are not being chaste. There must always be a threefold purpose to engaging the sex act; the third being the sanctification of the Sacrament. To do anything else is illicit, even inside of marriage.
I know that this sounds very foreign. I know that this isn't what is being taught today. However, if one really examines why the Sacrament exists and why the sex act is so scrutinized, one will see that the scrutiny from the Church isn't negative. It is positive. It is to protect the Sacrament of marriage, it is to protect the couple, and it is to protect the sex act itself from being cheapened. The sex act is more to the rational person than just a biological action. It is a rational way to bond. However, it is not the only way.
The Church has not always been 100% clear, but the reasoning is there. I would have you look at the Catechism of the Catholic Church starting at CCC #2360 and following. A couple of points to support what I've been saying:
The spouses' union achieves the twofold end of marriage: the good of the spouses themselves and the transmission of life. These two meanings or values of marriage cannot be separated without altering the couple's spiritual life and compromising the goods of marriage and the future of the family.
The conjugal love of man and woman thus stands under the twofold obligation of fidelity and fecundity.
Fecundity is a gift, an end of marriage, for conjugal love naturally tends to be fruitful. A child does not come from outside as something added on to the mutual love of the spouses, but springs from the very heart of that mutual giving, as its fruit and fulfillment. So the Church, which is "on the side of life," teaches that "it is necessary that each and every marriage act remain ordered per se to the procreation of human life." "This particular doctrine, expounded on numerous occasions by the Magisterium, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act."
Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast, "every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible" is intrinsically evil:
Thus the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality. . . . The difference, both anthropological and moral, between contraception and recourse to the rhythm of the cycle . . . involves in the final analysis two irreconcilable concepts of the human person and of human sexuality.
The Gospel shows that physical sterility is not an absolute evil. Spouses who still suffer from infertility after exhausting legitimate medical procedures should unite themselves with the Lord's Cross, the source of all spiritual fecundity. They can give expression to their generosity by adopting abandoned children or performing demanding services for others.
We must understand and remember that while the sex act is important and necessary, it is not an absolute. As the need arises or if circumstances determine, the Church certainly has always made provisions for other ways of fulfilling the Sacrament of Matrimony, beyond simply that of the sex act. It may be that those who are challenged with infertility may need to express their Marriage in a different way. And that is ok. There is nothing lost by the alternative, but rather there are Graces to be gained, just as in any relationship.
Bottom line, the Sacrament of Marriage isn't a license to "have sex." It is a commitment of responsibility to another. And that doesn't always nor should it always include "having sex."