IPA: /nɒnˈmɑːrɪtl/
KK: /nɑnˈmɑrtəl/
Not being in a marriage or not related to marriage.
They chose to live together in a nonmarital relationship.
Nonmarital → It is formed from "non-" (meaning not) and "marital" (from Latin "maritalis", meaning of marriage). The word "nonmarital" refers to something that is not related to marriage.
Think of 'non-' meaning 'not' and 'marital' relating to 'marriage' — so nonmarital means not related to marriage.