Should [marriage] be expanded to include same-sex couples with the state's imprimatur, it is conceivable that a meaningful percentage of heterosexual persons would cease to value the civil institution as highly as they previously had and hence enter into it less frequently, opting for purely private ceremonies, if any, whether religious or secular, but in any case without civil sanction, because they no longer wish to be associated with the civil institution as redefined, leading to an increased percentage of out-of-wedlock children, single-parent families, difficulties in property disputes after the dissolution of what amount to common law marriages in a state where such marriages are not recognized, or other unforeseen consequences."