Short answer
Anal intercourse is prohibited in Islamic law even between husband and wife. The existence of a valid marriage does not make every sexual practice permissible.
Islamic Guidance
What Islam teaches
The prohibition is established in prophetic narrations, and the mainstream position of Islamic jurisprudence considers anal intercourse prohibited between spouses.
Practical Guidance
What you can consider
If a spouse requests this practice, the other spouse may decline it on Islamic grounds. Couples should seek guidance from a qualified scholar if there is disagreement or uncertainty.
Sources
Sunan Abi Dawud; Jami' al-Tirmidhi; Sunan Ibn Majah.