'antimarital' originates from a combination of the prefix 'anti-' (from Greek 'anti' meaning 'against') and the adjective 'marital' (from Latin 'maritalis', related to 'maritus' meaning 'married').

'marital' comes from Latin 'maritalis' (from 'maritus'), passed into Old French and Middle English as 'marital' meaning 'pertaining to marriage'; the modern English formation 'antimarital' is created by attaching 'anti-' to that existing adjective.

The element 'marital' originally meant 'pertaining to marriage'; when combined with 'anti-' the compound meant 'against marriage' and has retained that negative/oppositional sense in modern usage.