Spiritually and legally, marriage has not taken place until the first act of sex between the new couple. Before the church became coy about such things, the nuptials were part of the wedding feast, and guests did not go home until they had been performed. This outward act by a man and woman is accompanied by an inward act of God, which fuses the two as one. This is where the sacrament of marriage occurs, and continues to occur, strengthening and confirming the marriage. Sex is necessary to a marriage, but the continuance of sex is not always necessary.
People become ill, incapable or simply too old to be bothered.
Although there is no biological reason why old age should prevent sex, many couples do, by mutual consent, cease to make love. Problems occur when sex ceases without mutual consent. This is usually a symptom that the marriage is ill, and if not treated it may well die. Marriage and sex can be compared to Baptism and Eucharist. Baptism is the sacrament which joins the believer to Christ, and Eucharist is the sacrament which confirms and strengthens that bond.