anl sex is prohibited insharia'h very strictly.

the holy prophet (peace be upon him said) that the person who will have anl sex (sex from back) with his partner Allah azzo jall will not look at him with mercy on the day of judgement(qayamat).

another hadith, the holy prohet (oeace be upon him ) said that the person who will have ansl sex with his wife is accused (maloo'n).

medically you can damage the useful tissu of wife which are very helpfull otherwise.and you can be sufferd from STD( sexual transmitted disease).AIDS is the disease which resulted through anal sex.

imam ghazali (AH)  said that anal sex is even worse than having sex durin menstruation because this filthy act cause undue pain and difficulty for the woman.