What is the ruling of Islam on marital infidelity for both sexes, whether it be a secret marriage or deviation?

Precise definitions of Shar‘i terms are essential for deriving rulings. The term "marital infidelity" might carry a problematic connotation if it replaces the Shar‘i terms.

If "marital infidelity" refers to: - Adultery (zina) or establishing forbidden relationships: This is unequivocally forbidden in Sharia by consensus (ijma‘). Its prohibition is mentioned in the Quran and Sunnah, as is the prohibition of unlawful gazing and impermissible seclusion (khalwa). - A man marrying another wife lawfully (Shar‘an) without the knowledge of his first wife: This is neither religiously forbidden nor an act of infidelity. However, he is obligated to act justly between his wives.