Libertarians already know it. Progressives hope libertarians won't show up when they are arguing with conservatives, because progressives don't want to admit that libertarians already know it. What’s the point of being at the forefront - by definition - if other people are already farther ahead on the issues?
What the libertarians already know and the progressives hope to forget is the history of the battle for marriage equality. The history that progressives hope people forget is why marriage licenses were instituted in the first place, because they were instituted specifically in order to deny them to people whom others did not want to be allowed to get married. In the era of Jim Crow that meant forbidding interracial marriages, and when those laws were initially passed progressives were quite in favor of them. The early libertarians, still called liberals in those days, were not because it was seen as a government intrusion into a private contract.
Eventually the civil rights movement started in the 1950s, not the 1960s as is commonly believed. One of the many struggles of that era was to end racial segregation in marriage. The battle took many years to complete, but eventually it was won and people of different races were allowed to marry. The libertarians had always been there fighting to end marriage discrimination. The progressives arrived late in the game, proclaimed themselves to be at the forefront, and claimed all the credit when marriage equality in race was finally achieved. Then they turned and proclaimed that libertarians are right wing conservatives in spite of being on the same side of that issue.
Those who were opposed to ending racial marriage discrimination would sarcastically ask what would be next, trying to invoke a slippery slope argument that ended with two people of the same gender getting married. Progressives, at the forefront by definition, laughed at it insisting that their current position was the goal and anything else was absurd. Libertarians, on the other hand, were quite willing to embrace same sex marriage even back then. Progressives didn’t want to talk to libertarians about it.
A generation later, and the progressives finally arrived at the position held by libertarians and proclaimed themselves to be at the forefront of the battle to grant marriage equality to same sex couples. Libertarians had always been there fighting to end marriage discrimination. The battle there is slowly being won, and the progressives are claiming all the credit for it happening. They are also proclaiming that libertarians are right wing conservatives in spite of being on the same side of that issue.
Those who are opposed to ending marriage discrimination based on gender sarcastically ask what is next, trying to invoke a slippery slope argument that ends with polyamorous relationships. Progressives, at the forefront by definition, laugh at it insisting that their goal current position is the goal and anything else is absurd. Libertarians, on the other hand, are quite willing to embrace polyamorous marriages. Progressives don’t want to talk to libertarians about it.
One could argue that the reason for the dissent is because the methods of ending inequality are so very different. Libertarians correctly point out that eliminating marriage licenses back in the days when the hot issue was racial marriage equality would have prevent the whole same sex marriage discussion and in the near future would have the same benefit on the polyamorous marriage discussion. Progressives do not want to be behind anyone else on any issue so insist that there is no future polyamorous discussion. Also they do not want to give up on marriage licenses, because even though the goal is to grant them to anyone it is still asking the state for permission to marry. It is very important to progressives that the state get the authority and respect it deserves.
So when the polyamorous discussion finally arrives, progressives will discover that they are suddenly at the forefront again, refusing to acknowledge anyone who was there before they were. And libertarians will correctly point out that if libertarians had been listened to back when the issue was race or gender then there would be no need for a polyamorous discussion.