In California, an alternative to marriage exists in the form of a Domestic Partnership.
Only two classes of people can register as domestic partners:
1) Those of the same sex that are at least 18 years of age; and
2) Those of the opposite sex in which at least one partner is over age 62.
Both parties wishing to form the Domestic Partnership must file a Declaration of Domestic Partnership with the Secretary of State (filing with the City or County will not suffice!) and both parties must meet the additional requirements of 1) Sharing a common residence; 2) not being blood related; and 3) not being in a marriage or domestic partnership with someone else.
Once a Domestic Partnership is registered with the State, the partners have the same benefits and responsibilities as to married persons, both during the partnership and after the partnership is dissolved by either dissolution or death.
Upon the filing of the Domestic Partnership registration, either or both parties may elect to change their middle or last name to reflect the new name they wish to be known by. This is not a requirement but rather is an option if desired by either party.
If you wish to know more about the process of registering as a domestic partner, see the website for the Secretary of State at https://www.sos.ca.gov/dpregistry.