Defense Of Marriage Act (DOMA). So-called Defense of Marriage laws (DOMAs) define marriage as a relationship between one man and one woman, for the purpose of excluding same-sex couples from the institution of marriage. There are two types of DOMAs: federal and state.he federal Defense of Marriage Act (DOMA) was enacted so that the federal government could restrict marriage to a union between a man and a woman. In so doing, the federal government was able to deny same-sex married couples any of the federal benefits opposite-sex married couples received.