From the New York Times:
Over tuna and turkey sandwiches, the advisers explained that New York’s Democratic governor was determined to legalize same-sex marriage and would deliver every possible Senate vote from his own party.
Would the donors win over the deciding Senate Republicans? It sounded improbable: top Republican moneymen helping a Democratic rival with one of his biggest legislative goals.
But the donors in the room — the billionaire Paul Singer, whose son is gay, joined by the hedge fund managers Cliff Asness and Daniel Loeb — had the influence and the money to insulate nervous senators from conservative backlash if they supported the marriage measure. And they were inclined to see the issue as one of personal freedom, consistent with their more libertarian views.
Within days, the wealthy Republicans sent back word: They were on board. Each of them cut six-figure checks to the lobbying campaign that eventually totaled more than $1 million.
In its take on the historic legislative action, New York magazine writes
How do you get rich Republicans to support gay marriage? By appealing to their libertarian sensibilities....Gay marriage has always been an ideal niche for liberaltarians. After all, it's the states, not the feds, that are the ones deciding whether it should be legal, a question that feeds into libertarians' federalist affinities. And when you strip away the cultural and identity politics, gay marriage is really just a fight about whether the government should be allowed to regulate personal liberty. On that...libertarians side with liberals.
Here's hoping the next step is drug legalization!