An-Nur (The Light)
Surah 24 · Verse 2
The adulteress and the adulterer, flog each of them (with) hundred stripes, and pity for them should not withhold you in (implementing) the law of Allah if you believe in Allah and the period hereafter, and let a party of believers witness their punishment.
Illegal sexual intercourse is a grave sin, therefore the punishment too is severe and is to be watched by others so that fearing this punishment they should keep away from it. A polytheist makes others to share the privileges of Allah (SWT) exhibiting divided loyalty. An adulterer makes another woman share the privileges of his existing wife, would be wife or ex-wife. It is probably in view of this it is stated that an adulterer should marry an adulteress i.e. a woman of this category or a polytheist woman similarly an adulteress.
