That is a bit of a generalization. Many libertarians are against the idea of government being involved in issues such as marriage at all. For exactly the reasons we are seeing now.
The more areas of life government pushes itself into, the more freedoms ordinary citizens lose. Expanding the class of people who can participate in some government controlled process does not expand freedom, but as bakers, pizza shop owners and others have learned, it takes away from their freedom.
Many libertarians support the right of people to do what they want with other consenting adults, so they would be opposed to laws forbidding a same sex couple from, for example, living together. But that isn't the same as "supporting" the re-definition of a cultural tradition to achieve some political goals.
Civil marriage has been the bedrock of civilization for thousands of years. Libertarians want it abolished, they agree with the radical left on this.