Marriage was instituted by God from the start of creation (Gn 1:27-28, 2:18-24). It represents a major part of the divine design for the good of persons - of the spouses and children - as well as of society.
For Christians marriage is much more. It is also a sacrament, one of those "efficacious signs of grace, instituted by Christ and entrusted to the Church, by which divine life is dispensed to us, [and which] bear fruit in those who receive them with the required dispositions" (CCC 1231). Marriage between Christians is therefore a source of grace. "Since it signifies and communicates grace, marriage between baptized persons is a true sacrament of the New Covenant" (ib. 1617).
Life is about getting to know and love God. Our starting point to know him is creation, particularly the masterpiece of visible creation: the human race. Each individual human, male or female, is made in the likeness of God. Man and woman each "images" God, in a different though complementary way. Considered together in their complementarity, they give a fuller image.
Although there is no sexuality in God, his creation of man as a sexually diversified being, also gives a key to what God is. An understanding of what it means to be masculine or feminine is essential in order to learn from a major revelation of himself inscribed by God into creation.
For centuries men and women have repeated the vow, "till death do us part", feeling that they express the natural resolve of two people who are so in love as to get married. The Catholic Church continues to take these words seriously; and sees them as corresponding to the fact that marriage is naturally meant to be an indissoluble union. She continues to teach the indissolubility of the marriage bond, not as a law of the Church applying just to the marriage of Catholics, but as a law of God for all marriages.
A man and a woman marry because they are in love and want to share life together. They choose one another in preference to all possible others, because they think they will be happier with the other than with anyone else. In many cases it works: but not always. People gradually run into difficulties, discover each other's defects, get irritated, have small quarrels and then bigger ones, feel attracted to someone else... Love and fidelity often survive the crisis, and take on a more voluntary and mature form. The couple "make it". Other couples do not; love declines and finally "dies". Then the idea of continuing in a loveless marriage appears senseless.