Deuteronomy 22:23

Betrothed. This case is treated as if it were literally adultery, inasmuch as the girl was pledged to her “husband” and regarded as a married woman. Compare the case of Joseph and Mary, whose marriage ceremony had not yet taken place, but only their betrothal, yet she was spoken of as his “wife” (Matthew 1:20, 24). The Western “engagement” falls short of the solemnity and binding character of the Eastern betrothal (see 2 Samuel 3:14).