It was left to the states. The states made a series of anti-gay amendments to their individual constitutions. These amendments were challenged. They worked their way up through local and state courts, to appeals courts to federal courts and ultimately to The Supreme Court of the United States. SCOTUS ruled them unconstitutional. Individual states (and their people) cannot legally enact unconstitutional laws.
THAT IS HOW IT WORKS.
Donald Trump fundamentally does not understand that, apparently. Nor does he seem to remember that as recently as a year ago, he called the whole thing a dead issue. He is clearly some combination of clueless, bullshitter, panderer and ignoramus. I'll let you decide on the percentages.
Donald Trump Says He Wants Supreme Court To Overturn Same-Sex Marriage
Donald Trump says he isn't a fan of the way the Supreme Court ruled on same-sex marriage.
"I would have much preferred that they ruled at a state level and let the states make those rulings themselves," Trump told Fox News. . .
Read more at: Newsy