Seeing all of these responses, I can’t help but think that a little exposition of Catholic doctrine might go a long way here, which I will do below.
Definition of Marriage: The lawful and life-long union of man and woman, instituted by God for the begetting and upbringing of children, as well as the mutual support and friendship of the spouses.
Primary Purpose of Marriage: The primary purpose of marriage is the begetting and upbringing of children.
Other Purposes of Marriage: Other purposes of marriage are the mutual friendship of the spouses, the support for one another as they grow old, and as a remedy for concupiscence.
Goods of Marriage: There are three goods of marriage: offspring, fidelity, and sacrament.
Sacramental Nature of Marriage: Marriage is one of the seven sacraments of the New Law. Every true marriage between baptized persons is a sacrament.
Matter of the Sacrament: The matter of this sacrament is the marriage contract itself.
Form of this Sacrament: The form of this sacrament is that which is laid down by the legitimate authority for contracting and solemnizing marriage.
Impediments to Marriage: Impediments to marriage arise either out of the nature of marriage (intrinsic impediments), or those imposed by the legitimate authority (extrinsic impediments). These impediments can be broken down into what were traditionally called diriment impediments, which render a marriage null, and so-called impeding impediments (redundant, I know), which render a marriage illicit, but not null. Furthermore, impediments can be distinguished as remote impediments, when the impediment is with who is contracting, and proximate impediments, when the impediment is with what is contracted. The three remote impediments of the natural law that render a marriage null are lack of the use of reason (violates the contractual nature of marriage), perpetual and antecedent impotence (physical inability to consummate the marriage; renders the fulfillment of the debt contracted impossible), and existing marriage (violates the properties of monogamy and indissolubility. The proximate impediments arising out of the natural law which nullify a marriage are: refusal to have children (violates the first good), intention to marry another (violates the second good), intention of not remaining married till death (violates the third good). Other impediments have been added by the legitimate authority. [Note: The distinction between remote and proximate impediments is my own, and does not belong to the corpus of canon law or Sacred Doctrine. Nevertheless, I have found it useful to distinguish thus.]
Legitimate Authority Concerning Marriage: Since marriage is a sacrament, the Church has the sole and exclusive right of regulating marriage between Christians, or at least Catholics.
This is the Catholic doctrine concerning marriage, both in itself and as a sacrament.
I hope this helps,
Benedicat Deus,
Latinitas