What was Herb's objection to his daughter dating Bobby Rupp?

Herb objected to his daughter's relationship with Bobby for a few different reasons. First, Nancy was only seventeen, and she been seeing Bobby for three years. He was the only boyfriend she'd ever had. Second, he thought they were too serious, and third, Bobby was a Catholic. The Clutters were Methodists. Because of this, the two would never be able to marry.

Mr. Clutter liked Bobby, and considered him, for a boy his age, which was seventeen, most dependable and gentlemanly; however, in the three years she had been permitted “dates,” Nancy, popular and pretty as she was, had never gone out with anyone else, and while Mr. Clutter understood that it was the present national adolescent custom to form couples, to “go steady” and wear “engagement rings,” he disapproved, particularly since he had not long ago, by accident, surprised his daughter and the Rupp boy kissing. He had then suggested that Nancy discontinue “seeing so much of Bobby,” advising her that a slow retreat now would hurt less than an abrupt severance later—for, as he reminded her, it was a parting that must eventually take place. The Rupp family were Roman Catholics, the Clutters, Methodist—a fact that should in itself be sufficient to terminate whatever fancies she and this boy might have of some day marrying.