Civil Unions
Some states allow for civil unions as an alternative to marriage. Although a civil union is not a marriage, laws allowing couples to form civil unions give those couples many of the same rights and benefits of spouses on a state level. However, other states are not required to recognize the civil union and the federal government does not provide all of the benefits, protections and responsibilities that are granted to married spouses.
The Supreme Court overruled the Defense of Marriage Act (DOMA) in Obergefell v. Hodges concluding that a denial of marriage rights to same-sex couples is unconstitutional. As a result, civil unions are less common as they were established as an alternative to marriage for same-sex couples which is now legal.