The human body takes on three postures most naturally; i.e. standing, sitting (or squatting) and laying down. As far as the positions of sexual intercourse are concerned, Islam has granted general permission provided that no unnatural act such as anal sex is performed.

How ever from medical point of view, sex in the standing position is undesirable (a complete chapter on this issue further ahead)

Thus two postures remain; the sitting or squatting posture and the sleeping posture. In this regard, there are certain subtle indications in the Quran and Hadith.
For example, in one verse of the Quran, Allah Ta’ala states:-
“It is He who created you from a single person, and made his mate thereof, in order that he may take comfort in her. Then when he covered her, she bore a light burden….”7/189
This is when a women lies in the supine position; while the man’s body lies over her in manner that he covers her body with his.
In the Hadith explaining the manner of ghusal, the following method of sexual union is alluded to:

“….When any of you sits between the four portions of a woman and then exerts himself upon her…..”
There is a difference of opinion as far as the interpretation of “four portions of a woman” is concerned. This seems to refer to the position in which a woman lifts her knees and the man enters her in a sitting/squatting posture, in which instance her thighs and calfs form the “four portions”. And Allah knows best.
Furthermore, any other posture or position, husband and wife desire to choose, is quite in order. Once Hazrat Umar (Radiyallahu-Anhum) had intercourse with his wife through rear entry (not anal entry). Later he was overtaken by the thought that perhaps he had committed an undesirable act. Immediately he rushed off to Rasulullah (Sallallahu-Aalyhi-Wasallam) calling out: “I have destroyed, I have been destroyed….! “He was asked what was the matter, to which he recounted his fear of having committed an undesirable act. Rasulullah (Sallallahu Alayhi-Wasallam) remained silent, giving no answer. Some time later, these verses of the Quran were revealed:

“Your wives are a tillage (farm) unto you, thus approach your tillage from wherever you wish”.-2/223.
Thereafter Rasulullah (Sallallahu-Alayhi-Wasallam) explained the purport of this verse: that intercourse is permissible in any position, from front or rear, so long as anal sex does not place, for that is haram. The example of the Quran for women is that of a farm; which can be approached from any direction, provided the seeds are planted only in the farm and not elsewhere! Similarly any method of approach is permissible, whether husband is on the wife or vice-versa, or whether they are on their sides or from the rear, whether sleeping flat or whether squatting, all positions are permissible, so long as his “seeds” are planted in the “farm” and not else where.
There was a false notion; a baseless superstition that the Jews of Madina entertained as far as rear entry was concerned. According to them, the child born out of such union would be squint eyed. Some Muslims were misled by this myth of the Jews. When the above-mentioned were verse of the Quran was revealed, all such false conceptions were shattered and demolished for once and all.