 As Finnis emphasizes, one feature of the new natural law account of marriage is that the structure of marriage can be fully explained by its purpose. Marriage is between one man and one woman because this is the unit able to procreate without third-party assistance; permanence is required to give children a lifelong family. Finnis charges, as noted above, that accounts which do not ground marriage in this purpose have no theoretical reason to resist the extension of marriage to polygamy, incest, and bestiality (Finnis 1995). As all non-marital sex fails to instantiate basic goods, there is no way morally to distinguish these different relations.