That is the libertarian party position, they do represent libertarian.
It is the usual stupid libertarian stuff, that ignores reality, to advance the left.
""1.3 Personal Relationships
Sexual orientation, preference, gender, or gender identity should have no impact on the government's treatment of individuals, such as in current marriage, child custody, adoption, immigration or military service laws. Government does not have the authority to define, license or restrict personal relationships. Consenting adults should be free to choose their own sexual practices and personal relationships.""
this goest to the lefts concept of postive and negtive right..
The libertarian position rule 1.3 is a classic libertarian negative right statement, government keep it nose out of this thing.
But Marriage is bit of positive right, the government gives you something, they license you and you get some privilege with that license
A libertarian should be for shrinking or eliminating a so called positive right not expanding a government positive right grant of privilege