"The Mettle of the Pasture" by James Lane Allen

Just before marrying, Rowan Meredith reveals to Isabel Conyers, his bride-to-be, that he had fathered an illegitimate child by a woman who refused to marry him. Isabel now refuses herself from marrying him because her "ideal" has been shattered. Because of pride Isabel won't tell anyone why she has thrown Rowan over but, later, when a scandal threatens Rowan, Isabel comes to his rescue.