Genesis 2:24

24 That is why a man leaves his father and mother and is united to his wife, and they become one flesh.

Read Genesis 2:24 Using Other Translations

Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.
This explains why a man leaves his father and mother and is joined to his wife, and the two are united into one.