"Fatwas" have been passed for its prohibition based on the following verse:
وَيَسْأَلُونَكَ عَنِ الْمَحِيضِ قُلْ هُوَ أَذًى فَاعْتَزِلُوا النِّسَاءَ فِي الْمَحِيضِ وَلا تَقْرَبُوهُنَّ حَتَّى يَطْهُرْنَ فَإِذَا تَطَهَّرْنَ فَأْتُوهُنَّ مِنْ حَيْثُ أَمَرَكُمُ اللَّهُ إِنَّ اللَّهَ يُحِبُّ التَّوَّابِينَ وَيُحِبُّ الْمُتَطَهِّرِينَ
"They ask you [Prophet] about menstruation. Say, ‘Menstruation is a painful condition, so keep away from women during it. Do not approach them until they are cleansed; when they are cleansed, approach them where Allah has ordained (مِنْ حَيْثُ أَمَرَكُمُ اللَّهُ). Allah loves those who turn to Him, and He loves those who keep themselves pure." (Qur'an 2:222)
Here the mentioning of "then (فَ) approach them where Allah has ordained / فَأْتُوهُنَّ مِنْ حَيْثُ أَمَرَكُمُ اللَّهُ ", is not forbidding anal sex as is claimed.
The context from which this part follows is menstruation which originates from the vagina. The verse states that when menstruation ends, vaginal intercourse can resume as the 'harm' the verse speaks of is no longer there. In explaining this, and since the vagina is specifically designed for penetration, Allah uses the term "where Allah has ordained" to refer to the vagina. This expression however does not mean He is prohibiting other sexual activities like anal penetration.
The fact that the anus' 'primary purpose' is not penetration does not make it prohibited for penetration. A body region or part does not need to have its 'primary' purpose sexual to be enjoyed sexually. For instance, one may argue the mouth's primary function is talking and eating, but it also plays sexual roles.
To put it another way, Allah uses the expression "where Allah has ordained" for the vagina because this is the only location whose primary purpose is penile penetration. And as stated, other regions may also be enjoyed sexually which do not necessarily have their primary purpose sexual - but perhaps secondary etc.
As an additional point, Quranic style of prohibiting something is not ambiguous, rather very clear and direct i.e. "forbidden for you are the catch of the land during pilgrimage..." (5:96), "Believers, do not consume usury..." (3:130). Allah is not embarrassed to state it in a clear manner, if He wished to prohibit it: "Allah does not shy away from telling the truth" (33:53).
In conclusion, this matter is a personal choice between a married woman and man - it is based upon love and mutual understanding of what they find and share as pleasurable in their intimacy - no prohibition can be found.
Caution! - Anus is a delicate tissue. Adequate care, generous/copious lubrication, and prolonged gentle foreplay in the anal area prior to any form of penetration is necessary with continuous communication - it is a delicate intimate act with the woman in control. Failure to do so can result in damage to the anus.
Note that the metaphor 'حَرْث / fields' used in 2:223 for wives, is still valid for vaginal intercourse. Forms of anal play can be seen as forms of pleasuring / enjoyment 'unrelated' to reproduction. Sexual enjoyments do not have to result in reproduction and the expression is used to refer to the reproductive element of men and women - not with the intention to restrict, but to express.
It is argued the Qur'an admonishes the people of Lot for engaging in this activity. The Qur'an is admonishing them for approaching the same gender sexually, and not the type of sexual activity. The Qur'an would be against any form of sexual activity between same gender, not restricted to this activity.
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