What is the ruling on a Muslim man who betrays his wife by establishing a relationship with another woman, and what is the ruling on a Muslim woman who establishes a relationship with a married man merely for entertainment?

Unlawful relationships between a man and a woman who are not mahram are a sin and an act of disobedience. Islamic law has forbidden them and blocked all paths leading to adultery, and the associated prohibitions such as khalwa (private seclusion), shaking hands, and gazing, as these are sins that lead to immorality. For a Muslim woman to establish an unlawful relationship with a man who is not mahram is among the gravest of sins and carries the most severe guilt, due to the resulting confusion of lineages and the husband's doubt concerning his children. Scholars, such as Sheikh Ibn Uthaymeen and Sheikh Al-Jibreen, have explicitly stated the prohibition of corresponding with non-mahram women due to the temptation and arousal of desires it entails, which can lead to immoral acts.