Vice President Joe Biden has said publicly that it is inevitable. President Barack Obama has not said so, but has acted as if it were. Obama did say in October 2011 that the U.S. Supreme Court would soon decide the issue of same-sex marriage.
Chances are that same-sex marriage will be gradually legalized, state-by-state, then with federal recognition, possibly one-by-one by federal appeals court districts, or possibly all at once by the Supreme Court. Finally all states and overseas territories will be required to recognize same-sex marriages and perhaps later, in a separate development, be required to permit them to be performed.
To learn from the past, one would be wise to look at the history of laws concerning interracial marriage.
A 2012 poll showed that 83% of US voters believed that same-sex marriage would be legal everywhere in the United States within five years.
As of December 20, 2013, about 33% of US states have legalized same-sex marriage.
Connecticut did on November 12, 2008. See related question below for a list of other states that legalized same-sex marriage afterwards.
Same-sex marriage has not been legalized in Laos.
No countries in the Balkans have legalized same-sex marriage.
No country in Central America has legalized same-sex marriage.
No former Soviet republics have legalized same-sex marriage.
The reasons appears to be cultural, political and religious.
No predominantly Muslim countries have legalized same-sex marriage.
the loving v. virginia decision. :) the actual question was supposed to be "which of the following legalized marriage in the entire united states between men and women of different races?"
Same-sex marriage was legalized in North Carolina on October 9, 2014.
Same-sex marriage was legalized in British Columbia on July 8, 2003.
Same-sex marriage was legalized in the Canadian province of Saskatchewan on November 5, 2004.