Same-sex couples are also disadvantaged when it comes to inheritance and estate taxes. Federal law generally allows married people to transfer an unlimited amount to their spouse at death without paying estate taxes — but not same-sex couples. That rule cost them about $3.3 million last year, according to The Williams Institute.
"The wealthier the couple is, the more costly it becomes," said Jill Hollander, a principal with the Financial Connections Group in San Francisco, who specializes in financial planning for gay couples.
And it's not just government policy. Extra costs can extend into every corner of financial life. Hollander recalled an insurance company that wouldn't recognize domestic partners under homeowners' policies, unless their names were listed on the title to property. That forced couples to also buy renter's insurance.
There are some steps same-sex partners can take to approximate the financial rights of married couples, but many don't think about the issue until they've been together for years.
Betsy Billard, a financial adviser with Ameriprise Financial, said often it takes a major event like a job change or the birth of a child, or some kind of conflict, to prompt couples to act.
"There are lots of couples that don't have the wherewithal to know they have to do it, or the financial ability to go to a lawyer and get it all set up," said Janet Chin of Oakland, Calif., who married Elinor Barron in 2008. "We're lucky enough that we have the finances to set up all the paperwork."
If it's early in the relationship, most professionals advise a couple to have an attorney draw up a cohabitation agreement, similar to a prenuptial agreement. "People don't want prenups because they're not romantic," said Richard Milstein, an attorney with Akerman Senterfitt in Miami. But having one can help avoid messy legal fights if there is a breakup and support what's in other documents, he said.
Other concerns to address include health care powers of attorney, child guardianship or adoption papers, deeds for homes, bank account and investment account paperwork and wills. Inheritance issues are particularly important, because the laws in some states could put blood relatives ahead of partners, especially if the state doesn't have legal marriage or civil unions.