But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good. Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural power and purpose sin against nature and commit a deed which is shameful and intrinsically vicious. Any use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life is an offence against the law of God and of nature, and those who indulge in such are branded with the guilt of a grave sin.
The person is thus capable of a higher kind of love than concupiscence, which only sees objects as a means to satisfy one's appetites; the person is capable rather of friendship and self-giving, with the capacity to recognize and love persons for themselves. (Pontifical Council for the Family: The Truth and Meaning of Human Sexuality, 1995).
Sex is only for marriage and marriage only for sex.
Nor are those considered as acting against nature who in the married state use their right in the proper manner although on account of natural reasons either of time or of certain defects, new life cannot be brought forth. For in matrimony as well as in the use of the matrimonial rights there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider so long as they are subordinated to the primary end and so long as the intrinsic nature of the act is preserved. (Papal Encyclical, Casti Connubii, 31 December 1930).
Sex is good. And if it is bad, it is because something else is wrong. The adult reality is that married people frequently do not want to or cannot have sex with each other. If, in theory, we can agree that having sex is better than not having sex, what do we do about that?