What is the ruling on one who deems permissible anal intercourse with a woman, and does one who deems permissible anal intercourse with a woman and sexual intercourse with a menstruating woman become an infidel, and what are the details of the ruling on infidelity in this regard?

Intercourse with one's wife in her anus is forbidden by the unanimous agreement of reputable scholars. Al-Shawkani stated in "Fath al-Qadir" that what was narrated from Ibn 'Umar and Al-Shafi'i regarding its permissibility is not authentically established, and that Al-Shafi'i explicitly prohibited it in six books. Whoever deems this act permissible, or intercourse with one's wife during her menstruation, while knowing of its prohibition, some scholars have stated that he has committed an act of disbelief (kufr), as mentioned by Ibn Rajab in "Fath al-Bari" and Al-Nawawi in "Sharh Muslim." Nevertheless, it must be understood that not everyone who falls into an act of disbelief has disbelief apply to them, and there are specific conditions for declaring someone a disbeliever.