Sebastián Piñera in June announced support for measure
South American country legalized civil unions in 2015
Vote took place two days after presidential election's first round
Voters in 2006 approved Marshall-Newman Amendment
'Marriage for All' statute received 64 percent support in referendum
National Assembly expected to vote on proposal in December
One president moved with caution, the other with speed
President Sebastián Piñera has urged lawmakers to support measure
Activists are celebrating Sebastián Piñera's historic change of heart
Lawmakers approved 'Marriage for All' bill in December