If the woman willingly agrees to her husband’s intercourse with her while she is menstruating, she is like him. But if he coerces her, then she bears no liability: neither sin nor expiation.

His statement, “and the anus,” means that intercourse with her in the anus is likewise forbidden—that is, inserting the penis into the anus—because Allah, Exalted is He, says: “Go to your tilth as you wish.” [al-Baqarah 2:223] The anus is not a place of cultivation. There are also several hadiths warning against it, whose combined strength indicates that they reach the level of sound hadith; among them is the Prophet’s ﷺ statement: “Indeed, Allah is not ashamed of the truth. Do not approach women in their anuses.” (1)

Moreover, sound analogy also requires this ruling. Excrement is undoubtedly more vile than blood. Thus, if Allah, Exalted is He, has forbidden intercourse with a menstruating woman because of the harm caused by blood, then anal intercourse is even more grievous and repugnant, for it resembles sodomy—sexual intercourse between males, Allah forbid. For this reason, some scholars called it “minor sodomy.” There is therefore no doubt that intercourse with a woman in her anus is forbidden.

As for enjoying her body between the buttocks, there is nothing wrong with that.

Shaykh al-Islam Ibn Taymiyyah, may Allah have mercy on him, said: Whoever is known to engage in anal intercourse must be separated from his wife—that is, the marriage must be annulled—because persistence in this sin, which is among the major sins, can never be condoned.