I think you are doing the math backwards. 13 states have legalized gay marriage. However, another 6 have legalized civil unions - this includes blue states like Hawaii an Illinois that would be very unlikely to pass a constitutional amendment. New Mexico has no position yet, leaving only 30 states that have banned gay marriage via statute on constitution.
And it only takes 13 of the 99 state legislature houses to block ratification.
There are 12 states that have legalized homosexual marriage:
Connecticut
*Delaware
Iowa
Maine
Maryland
Massachusetts
Minnesota
New Hampshire
New York
*Rhode Island
Vermont
Washington
There are 6 states that have civil unions but they* overlap with the homosexual marriage states:
Colorado
*Delaware
Hawaii
Illinois
New Jersey
*Rhode Island
There are 36 states that ban homosexual marriage. That leaves 2 states that have no ban or permit; New Jersey and New Mexico. NJ allows Civil Unions, NM does not.
I think the numbers are there for a Constitutional Amendment. The process itself would sober Americans up.