Prohibited Intercourse

Concerning the sensual relationship, these words of Allah Ta'ala were revealed: Your wives are a tillage to you, so go in to your tillage as you will, and send (ahead something) for your souls; and fear Allah and know that you will (one day) meet Him, and give glad tidings to the Believers. (2:223)

The Indian scholar, Waliullah Dehlavi, in explaining the Occasion of the revelation of this ayah and its significance says: The Jews had unnecessarily restricted the postures of sexual intercourse without any divine authority. The Ansar (of Madinah) being their friends, followed their practice and used to say, 'If a man has intercourse with his wife through the vagina while he is on her back, the child will have a squint.' Thus the verse, 'So go in to your tillage as you will' was revealed; that is, it is immaterial whether the husband is on top of his wife or on her back as long as intercourse is through the vagina, which constitutes the tillage. This is so because these matters have nothing to do with religious or social policy but are entirely a matter of personal taste. Such sayings were among the absurdities of the Jews and Allah Ta'ala abolished them. (Hujjat Allah al-Baligah, vol. 2, p. 134. )

It is not a function of religion to define the postures of sexual intercourse. However, a Muslim who fears Allah in his relationship with his wife and possesses the certainty that he will meet Him avoids the anus because the Prophet (peace be on him) said, "Do not approach women from the anus." (Reported by Ahmad, al-Tirmidhi, al-Nisai, and Ibn Majah.)
Again, he referred to such an act as "minor sodomy." (Reported by Ahmad and al-Nisai.) A woman of the Ansar asked him concerning vaginal intercourse from the back; he then recited to her, 'Your wives are a filth to you, so go in to your filth as you will,' but with only one receptacle. (Reported by Ahmad.)

'Umar came to him one day saying, "O Messenger of Allah, I am ruined!" "What has ruined you?" asked the Prophet (peace be on him). He replied, "Last night I turned my wife over," meaning that he had had vaginal intercourse with her from the back. The Prophet (peace be on him) did not say anything to him until the verse cited above was revealed. Then he told him, "From the front or the back, but avoid the anus and intercourse during menstruation." (Reported by Ahmad and al-Tirmidhi.)