Unsurprisingly, you still dont understand.
Marriage is indeed a right, per the case law already cited.
And as with all other rights, the right to marry is not absolute; a state may place restrictions on marriage provided those restrictions are applied to everyone equally in accordance with the Equal Protection Clause of the 14th Amendment.
Disallowing first cousins from marrying is Constitutional because its applied to everyone equally, no particular class of persons is singled-out for exclusion.
This is also why prohibiting same-sex couples from marrying is un-Constitutional: A State cannot so deem a class of persons a stranger to its laws. Romer v. Evans (1996). This would include marriage law as well.
Marriage is a fundamental right, subject to limitations by the government as is the case with all other rights; any limitations must be justified by objective evidence, be in accordance with due process and equal protection doctrine, and must further a proper legislative end.
Measures designed to restrict same-sex couples right to marry fail to meet any of the above criteria.