Second cousins dating each other in the United States is fine. They can get married to each other if they want to since it is legal in the U.S.
Some U.S. states do not allow cousin marriages, but this applies to first cousins and those having a closer relationship than second cousins. Examples of U.S. states that are strict on first cousin marriages are Ohio, Nevada, Indiana and Kentucky.
Indiana does not allow first cousins to marry in the state, but it welcomes first cousin couples who married elsewhere and decided to live in Indiana. Some U.S. states that allow first cousin marriages are Hawaii, Georgia, Alaska, Alabama, New York, Colorado and California.