Are the spouses considered to be in a state of zina (unlawful sexual intercourse) if one of them converts to Islam while the other remains in their non-Christian or non-Jewish religion, and their sexual relationship continues?

A Muslim is not permitted to remain in a marital relationship with a non-Muslim wife unless she is from the People of the Book.

If the wife is a polytheist (mushrikah), it is not permissible to remain with her, based on the Almighty's saying: "And do not marry polytheistic women until they believe," and "And do not hold on to the marriage bonds of disbelieving women."

However, if the wife embraces Islam while her husband is a disbeliever (whether he is from the People of the Book or not), the marriage contract is immediately annulled. She does not become lawful for him unless he embraces Islam during her 'iddah (waiting period).