Currently for a heterosexual couple to get married they receive permission from the state. Libertarianism being anti-state means the government should stick its nose into issues of major importance, such as property rights. Marriage is no such thing. It is decorative and wasteful on public finances to fund celebrants, write papers, etc. If government gets involved in marriage, we stand on a slippery slope that justifies government doing anything.
I recommend the full deregulation of the marriage market and the full privatization of firms that supply marriage contracts.
Under this system couples or polygamists can apply for marriage contracts from private institutions. This will lead to innovation in the marriage market, forcing firms to constantly reinvent new marriages, from gay marriage, to polygamist marriages, and even to marriage between different species. This innovation will bring out more varied consumption patterns and a boosting of GDP.