Prop8: simply incoherent?

From the LA Times, Dale Carpenter: (Read the whole thing, it's quite good).

So what potentially dooms Proposition 8 as it nears the Supreme Court is not necessarily the distinct whiff of prejudice but a lingering impression of incoherence. Despite what some critics last week charged, to challenge the sufficiency of the reasons offered for Proposition 8 is not to indict traditional marriage itself as bigoted and irrational. There are many rational, indeed compelling, reasons to support marriage between one man and one woman. Among others, getting heterosexuals to take responsibility for the children they conceive is a powerful reason to encourage them to marry. But encouraging heterosexuals to take seriously their familial obligations is no reason by itself to deny same-sex couples the full social and legal incentive to settle down. And to confer parental rights, marital rights and marital obligations on same-sex couples without giving them "marriage," Proposition 8's proponents must more convincingly answer one question. Why?