State laws

Cousin marriage is the only issue I can think of where the states are allowed to have their own policies and laws and the federal government remains neutral.  I believe that the Constitution guarantees a right to cousin marriage and most or possibly all of the founding founders would probably agree with me.  However, the second best outcome is to leave it as a state's rights issue.  Article Four, section 4, clause 1 of the Constitution says: "The United States shall guarantee to every State in this Union a Republican Form of Government".  The Ninth Amendment says: "The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people."  The Tenth Amendment says: "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."  Here are the laws for each state:

Alabama: Yes
Alaska: Yes
Arizona: Only if at least one is unable to reproduce
Arkansas: No, but sexual relations and cohabitation are allowed.
California: Yes
Colorado: Yes
Connecticut: Yes
Delaware: No, but sexual relations and cohabitation are allowed
District of Columbia: Yes
Florida: Yes
Georgia: Yes
Hawaii: Yes
Idaho: No, but sexual relations and cohabitation are allowed
Illinois: Only if both are over 50 or one is infertile
Iowa: No and it is unknown whether they permit sexual relations or cohabitation
Indiana: No, but sexual relations and cohabitation are allowed
Kansas: No, but sexual relations and cohabitation are allowed
Kentucky: No to marriage and no to sexual relations or cohabitation
Louisiana: No, but sexual relations and cohabitation are allowed
Maine: Proof of Genetic Counselling from a Genetic Counsellor
Maryland: Yes
Massachusetts: Yes
Michigan: No, but sexual relations and cohabitation are allowed
Minnesota: Only certain types
Mississippi: No to marriage and no to sexual relations or cohabitation
Missouri: No, but sexual relations and cohabitation are allowed
Montana: No, but sexual relations and cohabitation are allowed
Nebraska: No, but sexual relations and cohabitation are allowed
Nevada: No to marriage and no to sexual relations or cohabitation
New Hampshire: No, but sexual relations and cohabitation are allowed
New Jersey: Yes
New Mexico: Yes
New York: Yes
North Carolina: Yes
North Dakota: No to marriage and no to sexual relations or cohabitation
Ohio: No, but sexual relations and cohabitation are allowed
Pennsylvania: No, but sexual relations and cohabitation are allowed
Rhode Island: Yes
South Carolina: Yes
South Dakota: No to marriage and no to sexual relations or cohabitation
Tennessee: Yes
Texas: No to marriage and no to sexual relations or cohabitation
Utah: Only if both parties are 65 or older, or both are 55 or older with a district court finding of infertility of either party
Vermont: Yes
Virginia: Yes
Washington: No, but sexual relations and cohabitation are allowed
West Virginia: No, but sexual relations and cohabitation are allowed
Wisconsin: Only if the woman is at least 55, or either is permanently sterile
Wyoming: No, but sexual relations and cohabitation are allowed

Cousin marriage is banned in 25 states but allowed in 19 states and the District of Columbia.  4 states allow it if the cousins can't have any children, and two states allow it under special circumstances.