I want to know the forbidden sex practices in Islam, in the light of Qur'an and Sunnah. Why is anal sex forbidden? Can I touch the private parts of my wife while she is in menstruation periods?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Allah, The Exalted, has definitely prohibited anal sex. Fuqaha and doctors of medicine state that such an action has many harms. Among those are the following:
1) Turning one away from the natural way of having sex (vaginal intercourse) with a wife, one may become impotent to have the normal way of sex with her. This, in turn, stops procreation which is the main goal of marriage.
2) Negative influence on sexual organs, since anal sex may destroy seminal life and affect the structure of semen.
3) Producing bad effect on one's brain.
4) Loosening and rupture of rectum muscles.
As for touching a wife's private parts while in menstruation period, it is Haram if it is done without a cloth that prevents direct touch. This is the opinion of the majority of Muslim scholars, who legalize for a husband to take delight in his menstruating wife with the exclusion of what is between her knees and navel. It is recorded in Sahih al-Bukhari that 'Ayisha (Radiya Allahu Anha) said: "Allah's Messenger (Sallallahu Alaihi wa Sallam) used to order me, while in a state of menstruation, to put on an Izar (loincloth), and then, he took delight in me" . [Reported by Imam al-Bukhari ].
This means that one is not allowed to take pleasure in what is between the knee and navel of his wife while she is in her monthly period, since it is quite likely that one will commit a sin if there is no such reservation.
In other words, precaution necessitates prevention from touching the wife's private parts with the hand, penis or anything else as long as there is no cloth that prevents direct touching.
But, there is no harm to touch a menstruating wife's private parts as long as there is some loincloth, although it is better to avoid such an action, especially for one who cannot control himself in case of possibility of doing something Haram or when his lust reaches a climax. In connection with the above-stated subject, it is advisable to read our Fatwa: 81150 and 81381 about masturbation and homosexuality.
Allah knows best.