Allowing or not allowing cousins to get married is regulation, not definition. The definition of marrage is one man, one woman. As far as age, familial association, and race, those are details which are regulated. As far as the first cousin thing goes, if they are of a different sex, then it's a state issue. If they are of the same sex, it's a federal issue, because they do not fit the federal definition of marraige.
That is some tortured logic. The definition of marriage in NY is a union between a man and woman who may be first cousins. The definition of marriage in KY is a union between a man and a woman who may not be first cousins. The federal government recognizes both marriages, equally.
If California wants to recognize marriage between just two people, irrespective of gender, and you live in California and find that definition detestable, exercise your freedom to move, and leave the state.
"As far as the first cousin thing goes, if they are of a different sex, then it's a state issue. If they are of the same sex, it's a federal issue, because they do not fit the federal definition of marraige."
And, you base this on what particular section of the Constitution?