Sacrament of Matrimony

"Marriage, from its institution, should exist between two only, that is, between one man and one woman; that of two they are made, so to say, one flesh; and that the marriage bond is by the will of God so closely and strongly made fast that no man may dissolve it or render it asunder."

-Pope Leo XIII

See here: http://www.papalencyclicals.net/Leo13/l13cmr.htm.