Recent defenses of same-sex marriage and polygamy have invoked the liberal doctrines of neutrality and public reason. Such reasoning is generally sound but does not go far enough. This paper traces the full implications of political liberalism for marriage. I argue that the constraints of public reason, applied to marriage law, entail ‘minimal marriage’, the most extensive set of state-determined restrictions on marriage compatible with political liberalism. Minimal marriage sets no principled restrictions on the sex or number of spouses and the nature and purpose of their caring relationships, nor on which marital rights are exchanged, and whether they are exchanged reciprocally or asymmetrically. Minimal marriage supports adult care networks, urban tribes, friendships, and other forms of relationships as well as ‘traditional’ marriages. I provide a publically justifiable rationale for a legal framework supporting non-dependent caring relationships between adults. The argument is that caring relationships are primary goods, and that liberal justice accordingly requires legal frameworks supporting caring relationships. Minimal marriage is one such framework.