unequally yoked in the bible means interracial relations is a sin.
the darker races (mongoloids, negroids) never knew of christ, thus were unbelievers.
communion of lightness or the white christian race with the darkness or the unbelievers of the darker races, was forbidden.
2 Corinthians 6:14
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?