intermarriage (n.)

c. 1300, mariage, "action of entering into wedlock;" also "state or condition of being husband and wife, matrimony, wedlock;" also "a union of a man and woman for life by marriage, a particular matrimonial union;" from Old French mariage "marriage; dowry" (12c.), from Vulgar Latin *maritaticum (11c.), from Latin maritatus, past participle of maritare "to wed, marry, give in marriage" (see marry (v.)). The Vulgar Latin word also is the source of Italian maritaggio, Spanish maridaje, and compare mariachi.

Meanings "the marriage vow, formal declaration or contract by which two join in wedlock;" also "a wedding, the celebration of a marriage; the marriage ceremony" are from late 14c. Figurative use (non-theological) "intimate union, a joining as if by marriage" is from late 14c.

[W]hen two people are under the influence of the most violent, most insane, most delusive, and most transient of passions, they are required to swear that they will remain in that excited, abnormal, and exhausting condition until death do them part. [G.B. Shaw, preface to "Getting Married," 1908]

Marriage counseling is recorded by that name by 1939. Marriage bed, figurative of marital intercourse generally, is attested from 1580s (bed of marriage is from early 15c.).