Registered Domestic Partners
In CA there is a provision for filing the CA return for same sex couples as a joint return.
As a rule this saves the couple money. We do a fair number of these and so far they seem to be beneficial for the couple.
If you are a RDP, then you have to file a joint return or MFS for CA. MFS is not a desirable filing status so you don't want to do this.
There have been some recent changes in the way the IRS is viewing RDPs who are married. This seems to still be up in the air a bit, but if you are in this situation makes sure your tax preparer is up to date on this. If you prepare your own taxes, be careful in this area. Read up on it.