The antonyms for the phrase "sexual relationship outside of marriage" are varied depending on cultural and societal norms. Some antonyms are more conservative, such as "celibacy," "abstinence," "chastity," or "purity." Other antonyms are more liberal or progressive, such as "open relationship," "polyamory," "ethical non-monogamy," or "casual dating." Ultimately, the antonym for sexual relationships outside of marriage is subjective and dependent on an individual's values and beliefs. Whether one chooses to pursue a monogamous, non-monogamous, or abstinent lifestyle, it is essential to prioritize communication, consent, and respect in any romantic or sexual relationship.