When a man and a woman marry in the Catholic Church they promise to give themselves freely, fully, faithfully to each other until death and they agree to accept children willing from God and to bring them up in the faith.
The married couple themselves administer the sacrament of matrimony to one another. Sacrament is understood as "a visible sign of an inward grace." Married life is sacramental because the love of God is revealed through the married couple's love for one another. Whether we look at the marriage ceremony or the life that unfolds from it, the man and woman's gift of each to the other is the sacrament. It is the place where God appears.
In giving themselves freely a person is saying "it is my choice to spend the rest of my life with you. No one and no situation is coercing me to do it." Therefore a young couple cannot be expected to marry because of a pregnancy or other external pressure. The Catholic Church does not approve "arranged" marriages which deny a person the freedom to say ‘yes' or ‘no'. Parents may not exert undue pressure on young people to marry. Similarly couples should not marry because they feel "this may be our last chance" or because one or other of them is dependent on the other. It should not be a manipulative relationship.
In giving themselves fully to each other, husband and wife are not just offering "to meet you half way". It is not a 50/50 relationship. They are saying I want to live my life for you and for your salvation. It is a 100/100 relationship which finds its ultimate expression in the physical self-giving of their love that holds nothing back. They do not practice contraception because that would be to refuse to share a part of themselves - their fertility - with each other. If at some time in their marriage they feel it is practically unreasonable to have another child at that time they may practice natural family planning to avoid pregnancy for a time. But their love for each other is always "open to life" in all its fullness.
Their love for each other is exclusive. This means that they will not have a similar relationship with another person while they are both still alive. Sometimes, very sadly, a couple may find that for some reason they are no longer able to continue to live together in marriage - this may be due to physical or mental illness or other serious and irrevocable reason, but having promised to be "faithful until death do us part" they may not enter into another marriage while their spouse is still alive. Of course married people can and will have friendships with other people, but these relationships will not have the close and intimate nature of a marriage. The Church does not accept pre-marital or extra-marital affairs.
In the past, the promise that husband and wife would love each other forever was expressed like this: "I promise to love and honour you, in sickness and in health, for richer for poorer, to love and to cherish until death do us part". While those words are not so often used these days, the Church's understanding of the vocation to marriage remains the same. Read the testimonies of some married couples.