Why are Anti-Gay marriage people so nostalgic about "traditional marriage" when?

You need to do some serious research on the definition of "traditional Marriage" before you define it as you did in your question. Traditional marriage is an agreement between a male and a female to join together in Holy Matrimony performed by a pastor/priest/clergy and/or a secular legal agreement performed by a Justice of the Peace. The Holy Bible instructs the male to love his wife as Christ loves the church and that the female should be submissive to her husband. Christ gave his life for the church because he loved it so much. Get the picture. Male & male unions and/or female & female unions are odd, unnatural, out of the ordinary, queer, unusual and are unable to produce offspring naturally. There may have been some incidents in ancient history of which you speak, however, they were not considered traditional by any means by anyone in mainstream society.