Qur'aanic Verse 2:222:
222. Wayas-aloonaka AAani almaheedi qul huwa athan faiAAtaziloo alnnisaa fee almaheedi wala taqraboohunna hatta yathurna fa-itha tatahharna fa/toohunna min haythu amarakumu Allahu inna Allaha yuhibbu alttawwabeena wayuhibbu almutatahhireena
222. And they ask you about menstruation. Tell them, "It is harmful.412, 413 So keep away from women in menstruation, and approach them not till they're cleansed.414 And when they are cleansed, go to them in the manner decreed for you by Allah.415 Indeed, Allah loves those who repent and seek forgiveness, and He loves those who keep themselves clean."416
412. Every month the uterus of an adult woman, who is not already pregnant and who has not reached the age of menopause, prepares itself for pregnancy. It prepares itself to welcome, nourish and help develop an impregnated egg for over nine months to transform it into a human form ready to come out into the open world. The preparation results in a thickened lining of the inner wall of the uterus.
413. But if the pregnancy does not happen, the thickened lining of the uterus wall gets damaged and is shed out through the vagina in the form of tissues and blood. This draining of the damaged uterus lining is known as menstruation, menses or 'the monthly period' in women. A menstruating woman could be in a vulnerable state, both mentally and physically.
414. Recent studies have revealed that there is a greater risk of transmitting AIDS and other sexually transmitted diseases through sexual intercourse during menstruation. The onslaught of AIDS in recent times could be a direct consequence of the ignorance of or blatant disregard for this divine guidance/command. Man's welfare lies in the obedience of Allah's directives.
415. Aberrations in sexual behaviour like indulging in anal sex, being unnatural and therefore not as per the divine design, are obviously not as decreed by Allah.
416. The All-knowing Allah has made the sex urge strong, obviously for the purpose of the procreation of mankind till the Last Day. It is so strong that even the pious may be tempted to taste the forbidden fruit, sometimes. That is why Allah promises forgiveness for those who may commit a sin in this regard, unintentionally, and then repent and sincerely resolve not to commit the sin again. Please also take note of the importance given to maintenance of cleanliness in general.
An extract from soon-to-be-published, inshaAllah, Qur'aanic Studies.