Chapter XLVIII: Section 2: The Essence and Purpose of Marriage

There is an obvious distinction between the _essence_ of a thing and the _ends_ or purposes for which the thing exists. For example, in a business partnership the _essence_ of the partnership is a legal instrument, whereas the _purposes_ or _ends_ of the partnership are various commercial projects. The following is a clear statement, by Father Vincent McNabb, O.P., [124] of Catholic teaching concerning the nature and end of marriage:

"Marriage is an indissoluble state of life wherein a man and a woman
agree to give each other power over their bodies for the begetting,
birth, and upbringing of offspring. The natural and primary end of
marriage is this duty towards offspring. But, as sin has despoiled the
human will and disturbed human relations, marriage has now the
secondary end of allaying sexual lust.

"But it is a principle of ethics that what is primary cannot be set
aside as if it were secondary, nor can the secondary be sought as if it
were primary. To invert the ethical order is to bring in that disorder
which is called sin. If the human act brings in a slight disorder, it
is venial sin; if the human act brings in a grievous disorder it is a
grievous or mortal sin.

"It is a grievous disorder, and, therefore, a grievous sin, to desire
satisfaction in such sexual intercourse as could not result in the
begetting of offspring.

"As the wedded pair have given each other power over their bodies it
would be a grave sin for one to refuse either altogether or for a
considerable time the fulfilment of the marriage debt. But it is not a
sin if by mutual agreement the wedded pair refrain from the marriage
debt for a time, or for ever. As a rule, and speaking objectively, it
would be heroic virtue for a wedded pair to abstain for a long time,
and still more for ever, from the marriage debt. To counsel such a
practice indiscriminately would be a sinful want of prudence, and, in a
confessor, of professional knowledge.

"It is quite clear that by mutual consent, even without any further
motive, the wedded pair can abstain from marital intercourse. Still
more may they abstain for a time or for ever, for a good motive, e.g.
in order to have time for prayer, for good works, for bringing up such
family as they already have to support."