Catechism in a Year
Tuesday, November 06, 2018
347. What sins are gravely opposed to the sacrament of Matrimony?
Adultery and polygamy are opposed to the sacrament of matrimony because they contradict the equal dignity of man and woman and the unity and exclusivity of married love. Other sins include the deliberate refusal of one's procreative potential which deprives conjugal love of the gift of children and divorce which goes against the indissolubility of marriage.
Further reading: CCC 1645-1648
348. When does the Church allow the physical separation of spouses?
The Church permits the physical separation of spouses when for serious reasons their living together becomes practically impossible, even though there may be hope for their reconciliation. As long as one's spouse lives, however, one is not free to contract a new union, except if the marriage be null and be declared so by ecclesiastical authority.
Further reading: CCC 1629, 1649
These excerpts are from the Compendium of the CCC.