Race is an innate characteristic - sexual preference, a chosen behavior, is not.
Race is protected from discrimination according to the Constitution - sexual preference is not.
Marriage may be a "fundamental right," but marriage is, by definition, the union between a man and a woman.
Everyone has the same right to marry the person the choose so long as both persons meet the same requirements: opposite sex, legal age, consent and unrelated by blood. If changing one of these requirements, to suit the demands of sexual perverts, is acceptable, what preserves the other requirements from similar change?