Civil marriage equality in effect at the state level since 2013. (Same-sex couples may legally marry.)
In addition to marriage, Hawai'i also allows both same-sex and opposite-sex couples to formalize their relationships legally in the form of civil unions and reciprocal beneficiary relationships. Civil unions provide the same rights, benefits, and obligations of marriage at the state level, while reciprocal beneficiary relationships provide only a limited set of rights.
Hawai'i holds an important place in the history of the marriage equality movement. For many people in the American LGBTQ community the first time they ever seriously thought about possibly gaining the right to civil marriage was when Hawai'i began to fight for marriage equality. Baehr v. Miike (originally Baehr v. Lewin) was a lawsuit in which three same-sex couples argued that Hawai'i's prohibition of same-sex marriage violated the state constitution. Initiated in 1990, the passage of an amendment to the state constitution banning marriage equality in 1998 led to the dismissal of the case in 1999. The publicity surrounding this case had powerful ramifications through-out the United States.
When the state legislature was in the process of passing the Hawai'i Marriage Equality Act in 2013, legislators went out of their way to ensure that everyone who wished to could speak to the bill. The result was an extensive "citizens' filibuster" attempt to block the bill's progress - over 5,000 people spoke. Of note, is that the large majority of people who spoke in opposition said they did so because of their Christian religious beliefs. These were primarily people who identified themselves as native Hawai'ians, crediting their opposition to marriage equality to a religion that had been imposed on the islands by outsiders in the 1800's. None of the opposition seemed to represent non-Westernized Hawai'ian culture.