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.The California platform (pdf) has more details:
13. MarriageSo they want to allow polygamous and incestuous marriages. The state would not really be out of the marriage business, because it would have to recognize and enforce those marriage contracts.
We support the rights of individuals to form private relationships as they see fit, either by contract or by mutual agreement. We regard marriage as one such private relationship. The State of California should not dictate, prohibit, control, or encourage any such private relationship. To implement this principle, we advocate:
A. The repeal of all marriage and marriage dissolution laws and their replacement by contracts where desired by the parties.
B. Property not specified as "community property" not being presumed as such.
C. The repeal of all alimony laws.
D. The recognition in law of marriage contracts as an addition to, or replacement for, marriage and marriage dissolution laws.
E. The right of all consenting adults to form marriage contracts without regard to gender, sexual preference, degree of consanguinity, or number of parties to said contracts.
F. Until such time as the state of California ends its involvement in marriage, we call upon the state to issue marriage licenses to any adults without regard to gender.
This approach presumes that the only interested parties are consenting adults. But the main point of marriage law is to protect the interests of the resulting kids, and they are certainly not consenting adults to the arrangement.
I suppose that a couple could agree to a marriage contract, and make their own decision about community property. But the socially more important part is to agree on the care and custody of any resulting kids. For example, they may wish to agree to joint child custody in case of divorce, and that they will separately support the kids.
Unfortunately, the laws of all 50 states prohibit enforcement of any such contract. Child custody is put at the discretion of a family court judge, and child support is based on formulas related to the income tax system. The court will not enforce any contract for the care and custody of kids. All parents are subject to having their lives micromanaged by judges who have opinions about the best interest of the children.
California already lets consenting adults behave as they wish. If the Libertarians really followed their principles of minimum government and maximum freedom, then they would support parents making enforceable contracts for the care and custody of their kids. I could be wrong, but I do not see where the Libertarian Party supports this basic freedom.