MANCHESTER, N.H. — A question about same-sex marriage from a Vietnam veteran led to a heated exchange with Mitt Romney on Monday morning as Mr. Romney was making the rounds at a restaurant here. After receiving the endorsement of Manchester’s mayor, Ted Gatsas, Mr. Romney greeted diners who were finishing up their breakfasts at Chez Vachon, and took a seat next to Bob Garon, 63, a veteran from Epsom, N.H. “I have a question for you,” said Mr. Garon, who was wearing a Vietnam veteran hat and a red flannel shirt with a pack of Basic cigarettes tucked into the...