What is the ruling of Islam concerning a husband or wife who engages in a direct or indirect sexual relationship with another party, and does this annul the marriage contract?

Marriage protects a person from disobeying Allah. The forbidden act between spouses is a disobedience to Allah Almighty; it is a major sin if it is adultery and a minor one if it is less than that. Sincere repentance for it is obligatory. The marriage contract is not annulled because of this, but the husband has the right to divorce his wife, and the wife has the right to refer the matter to a judge to annul the marriage if the husband insists on committing the sin.