On May 17, 2004, Massachusetts became the first state to issue marriage licenses to same-sex couples. Less than a decade later, on Aug. 29, 2013, the Internal Revenue Service announced that all legally married same-sex couples in the U.S. will be allowed (and required) to file joint federal income tax returns, regardless of state of residence.

Same-sex marriages are now taking place in Maine, Maryland, Washington, Colorado, Rhode Island, Delaware, Minnesota and some counties of New Mexico. This year, the U.S. Supreme Court ruled unconstitutional Section 3 of the federal Defense of Marriage Act (DOMA), which had barred federal recognition of same-sex marriage. It also overturned California's Proposition 8, which made same-sex marriage legal in the U.S.'s largest state by population.

In total, 13 states and Washington, D.C., now recognize same-sex marriages, and another seven states allow civil unions or domestic partnerships. In a short time, nontraditional couples have grown from a small, controversial niche into an important segment of the U.S. mainstream. Financial advisors who recognize the opportunity, and stand ready to meet it with advice and service, will be positioned to capture long-term growth.