Tradition can be a good thing -- but not when it is just a cover for bigotry. The truth is that there is no real traditional definition of marriage. It has differed in different countries, and at different times throughout history. Even the Bible (which is commonly used to discriminate against same-sex couples) has not always supported the current idea of a heterosexual union of one man and one woman because of love (as a cursory reading of the Old Testament will show).
In the past, the false argument of "tradition" was used to discriminate against persons of a different race, ethnicity, or skin color. Today that same ridiculous argument is being used to discriminate against the LGBT community. And whether those using this failed argument want to admit it or not -- it definitely is BIGOTRY.