As of 2013, four states, Colorado, New Jersey, Hawaii and Illinois, allow civil unions for same-sex couples. Hawaii and Illinois also offer civil unions to opposite-sex couples as well. Civil unions, like domestic partnerships, provide same-sex couples with similar, but not identical, rights to marriage. You must be 18 years old and mentally competent to be part of a civil union. A civil union is between two people, you cannot be a part of more than one civil union or in a civil union with a close relative.
State civil union partners have priority inheritance rights and can make emergency medical decisions and funeral arrangements for partners. They can own joint property, like married couples, and generally can jointly-adopt children. They are entitled to custody and support following breakups, and are protected under domestic violence statutes. State employees may add partners to medical and pension benefits. Partners can file joint state tax returns and cannot be forced to testify against each other in state court.
States offering civil unions generally honor out-of-state legal unions. For example, if you are civilly united in Illinois and relocate to Honolulu, the state of Hawaii legally recognizes your civil union. Or, if you are a same-sex married couple in Massachusetts, or have a Nevada domestic partnership, Colorado considers your out-of-state union valid. New Jersey requires you to reaffirm an out-of-state civil union by filing a formal application and wait 72 hours for issuance of a New Jersey civil union license.
As of 2013, most states do not offer any forms of legal unions for same-sex couples. States that do not provide for civil unions, domestic partnerships or same-sex marriage do not recognize same-sex unions from other states. Therefore, if you and your same-sex partner enter a civil union in New Jersey, a state like Michigan that bans all forms of same-sex unions will not recognize your union or any of its benefits.
Civil unions do not offer the same federal benefits as marriage. For example, a civil union does not entitle you to your partner's social security or federal government disability benefits. You may not file federal joint tax returns like married couples, nor are you eligible for marital inheritance tax breaks if your partner dies. Moreover, an immigrant who enters a civil union, rather than a marriage, with a US citizen does not receive an automatic green card.