Follow these simple guidelines and you're sure to dodge trouble and get a (figurative) U.S. governmental stamp of approval on your wedding certificate from another country.
The U.S. State Department recognizes that a marriage abroad is valid if you follow the country's laws in which you are married. No American diplomat or consular office representative need be present at the ceremony. If you have any questions, you should direct them to the attorney general of the state in which you and your spouse will reside. Big caveat: If you are applying for a foreign nationality of another country (naturalization) you may lose American citizenship status. See the U.S. State Department's rules on loss of U.S. nationality in the link below.
For more information on U.S. citizens getting married abroad, visit the U.S. State Department's site.