But mutual faithfulness is the language of marriage. Mutual faithfulness is not a matter of pragmatics, it is a matter of morality.
Marriage is the moral context for sex.
Safe sex is moral sex. Marriage was instituted in Creation. God created man male and female commanding them to be fruitful and multiply. Marriage was given to provide companionship, as the context for sex and procreation, and to be the nucleus of social order. Marriage is inherently a sexual relationship in that it is the union of the two sexes.