While you are waiting for the evidence you have asked for, you can answer to your own unsupported claim.
Who are these many libertarians and what is their good argument to oppose gay marriage?
As has been established, the
official libertarian platform condemns opposition to gay marriage.
Platform - Libertarian Party
1.3 Personal Relationships
Sexual orientation, preference, gender, or gender identity should have no impact on the rights of individuals by government, such as in current marriage, child custody, adoption, immigration or military service laws. Consenting adults should be free to choose their own sexual practices and personal relationships. Government does not have the authority to define, license or restrict personal relationships.
And even further...
3.5 Rights and Discrimination
We condemn bigotry as irrational and repugnant. Government should not deny or abridge any individual's rights based on sex, wealth, race, color, creed, age, national origin, personal habits, political preference or sexual orientation. Parents, or other guardians, have the right to raise their children according to their own standards and beliefs.