The terms used to designate civil unions are not standardized, and vary widely from country to country. Government-sanctioned relationships that may be similar or equivalent to civil unions include civil partnerships, registered partnerships, domestic partnerships, significant relationships, reciprocal beneficiary relationships, common-law marriage, adult interdependent relationships, life partnerships, stable unions, civil solidarity pacts, and so on. The exact level of rights, benefits, obligations, and responsibilities also varies, depending on the laws of a particular country. Some jurisdictions allow same-sex couples to adopt, while others forbid them to do so, or allow adoption only in specified circumstances.

As used in the United States, beginning with the state of Vermont in 2000, the term civil union has connoted a status equivalent to marriage for same-sex couples; domestic partnership, offered by some states, counties, cities, and employers since as early as 1985,[6] has generally connoted a lesser status with fewer benefits.[7] However, the legislatures of the West Coast states of California, Oregon and Washington have preferred the term domestic partnership for enactments similar or equivalent to civil union laws in East Coast states.

In certain territories which allow for some degree of government-recognized same-sex unions, the reduced rights and responsibilities of such unions are also afforded to opposite-sex couples. France's civil solidarity pacts (PACS) has become increasingly popular among opposite-sex couples with them accounting for 95% of all PACS. In some jurisdictions such as California and Washington domestic partnerships are available to a opposite couples if one of the parties is over the age of 62. In the District of Columbia and Hawaii domestic partnerships are available to non-conjugal partners such as relatives.

Civil unions are not seen as a replacement for marriage by many in the gay community. "Marriage in the United States is a civil union; but a civil union, as it has come to be called, is not marriage," said Evan Wolfson of Freedom to Marry.[8] "It is a proposed hypothetical legal mechanism, since it doesn’t exist in most places, to give some of the protections but also withhold something precious from gay people. There’s no good reason to do that." However, some supporters of traditional marriage view the matter differently; Randy Thomasson, Executive Director of the Campaign for California Families, calls civil unions “homosexual marriage by another name” and contends that civil unions provide same-sex couples “all the rights of marriage available under state law.”[5] The California Supreme Court, in the In Re Marriage Cases decision, noted nine differences[9] in state law.

Civil unions are commonly criticised as being 'separate but equal', critics say they segregate same-sex couples by forcing them to use a separate institution. Supporters of same-sex marriage contend that treating same-sex couples differently from other couples under the law allows for inferior treatment and that if civil unions were the same as marriage there would be no reason for two separate laws. A New Jersey commission which reviewed the state's civil union law reported that the law "invites and encourages unequal treatment of same-sex couples and their children".[10] Some have suggested that creating civil unions which are open to opposite-sex couples would avoid the accusations of apartheid.[11] These have still been criticised as being 'separate but equal' by former New Zealand MP and feminist Marilyn Waring as same-sex couples remain excluded from the right to marry.[12]

Proponents of civil unions say that they provide practical equality for same-sex couples and solve the problems over areas such as hospital visitation rights and transfer of property caused by lack of legal recognition.[13] Proponents also say that creating civil unions is a more pragmatic way to ensure that same-sex couples have legal rights as it avoids the more controversial issues surrounding marriage.

Many supporters of same-sex marriage state that the word 'marriage' matters and that the term 'civil union' (and its equivalents) do not convey the emotional meaning or bring the respect that comes with marriage.[10][14] Former US Solicitor General and attorney in the Perry v. Schwarzenegger case Theodore Olsen said that recognising same-sex couples under the term 'domestic partnership' stigmatizes gay people's relationships treating them as if they were "something akin to a commercial venture, not a loving union".[15] Many also contend that the fact that civil unions are often not understood can cause difficulty for same-sex couples in emergency situations.[16]

Some commentators such as Alan Dershowitz have suggested that the best way to deal with the controversy over same-sex marriage would be to remove marriage from civil law and have all marriages become civil unions.[17] Following the passage of Proposition 8 which banned same-sex marriage under in California's constitution, two college students attempted to enter a ballot iniative which would have removed marriage from California law and replaced them with domestic partnerships. The initiative didn't receive enough signatures to make it onto the ballot.[18] In Sweden following the governing coalition plan to pass a gender-neutral marriage bill the conservative Christian Democratic Party proposed an alternative law which would have removed the word 'marriage' from Swedish law, however this was rejected and the bill allowing same-sex marriage was passed.[19]