Pope Benedict XVI argued that the reasons against marriage are "inscribed in human nature itself." The argument that something is desirable merely because it is natural, or undesirable merely because it is unnatural, is called the "appeal to nature," and any class in logic will tell you that it is a fallacious argument.
The values one seeks in nature are really your values, not nature's. For example, an advertiser may claim that its product is "all natural," implying that it is environmentally friendly or safe. However, many toxic substances are "all natural," including lead, hemlock and poisonous mushrooms.
The pope's assertion of "the natural structure of marriage as the union of a man and a woman" is laughable. Marriage is not in nature; it is a human social construct that has changed as human values have changed.