I'm from Arkansas and I know next-to-nothing about California law. However, according to the information on the California Secretary of State's website, "persons of opposite sexes may not constitute a domestic partnership unless one or both of the persons are over the age of 62." The domestic partnership status seems to be for those who cannot get married due to laws against same-sex marriages and also for those (of the opposite sex) who would be placed in extreme economic hardship by getting married due to the loss of Social Security/retirement benefits. Here is the website I looked at: http://www.ss.ca.gov/dpregistry/
It seems to be like the regulations of most corporations that offer domestic partnership benefits: if you CAN get married, then that is what you have to do. If you CAN'T, then you can register as domestic partners. Hope this helps.