Your question should be divided up into two questions. First, is there a genetic reason why you shouldn't be in a relationship? Second, is it socially acceptable to be in such a relationship.
The answer two the first question is pretty easy. No. Unless your family has a history of some rare genetic diseases, there is no reason why you shouldn't have sex and possibly have children with your second cousin. Hell, even your first cousin is fair game as long as the incest doesn't happen over multiple generations. As mentioned earlier, having a children with a first cousin is about as risky as having children with someone over 35.
The answer to the second question is subjective. I think dating second cousins is socially acceptable because I do not know any of my second cousins. I would be more weirded out if it was someone I knew as family my entire life and then started dating, then someone who turned out to be distantly related to me. That said, my family reunion wouldn't be the first place I would be looking for a date.