CINCINNATI — No one heckled Senator Rob Portman during his first appearance before thousands of Ohioans since becoming the most prominent elected Republican in the country to endorse same-sex marriage. “Wouldn’t you say,” he said, turning to his wife, Jane, after marching in an opening day parade for the Cincinnati Reds on Monday, “that most of the comments were thank yous and thumbs up and ‘You did the right thing’?” True, but for some of the Ohioans who acknowledged Mr. Portman, doubts flickered below the surface. “Senator!” said one spectator, Pete Kidnocker, reaching out to shake Mr. Portman’s hand. But...