Clayton's description would INCLUDE blood relatives since he keeps using the term consenting adults. How again can they be discriminated against if no class of consenting adult may be so discriminated against for any reason?
Ive made no consenting adults argument.
I simply restate the Constitutions requirement that all laws be applied consistently predicated on a rational basis.
Laws banning blood relatives marrying is rational, as no class of persons is being singled-out for exclusion, it applies to all races, genders, ethnic groups, and sexual orientations equally. Such laws are consequently Constitutional.
As WorldWatcher correctly notes, however, there is no rational basis or compelling reason for the state to deem a class of persons a stranger to its laws, and forbid homosexuals access to marriage.
Religious objection is certainty not rational. It may also be in violation of the First Amendment should the state attempt to use such a rationale to justify exclusion. In addition, that marriage has traditionally been between men and women is also not Constitutionally valid. As the Court observed in Lawrence:
[T]he fact a States governing majority has traditionally viewed a particular practice as immoral is not a sufficient reason for upholding a law prohibiting the practice
LAWRENCE V. TEXAS