In August, the California Teachers Association spent $250,000 to defeat the measure to ban same-sex marriage and recently spent another $1 million.
The union maintains it's a civil rights issue.
"This is about the right of people to make their own decision as protected by the California State Constitution," said Dennis Kelly from United Educators San Francisco.
Kit Greerty is an elementary school teacher in Martinez. She's upset her union dues were spent to try to defeat Proposition 8.
"It's $1 million dollars that could be better spend on teacher recruitment, curriculum reform and needs of our special kids," said Greerty.
But the union says it had to respond to recent ads paid for by the "Yes on 8 Campaign." The union calls them misleading.
"The teaching of marriage is not part of the curriculum, that is simply not there. So when people raised that as a concern, that was something we thought people needed to know," said Kelly.
The state Superintendent of Public Instruction also responded with a new ad.
The decision to oppose Proposition 8 was made back in June in Los Angeles. CTA's 800-member policy body voted to support or oppose a number of propositions on the November ballot.
So far, most of that union money has been spent on the effort to defeat Proposition 8, more than on any other measure this year.
But in the past, the union has spent lots more -- $60 million in 2005 defeating governor Arnold Schwarzenegger's three ballot initiatives. Those measures had to do with teacher tenure, school funding and ironically, employee consent for political contributions.
To learn more about some e-mails circulating about a school boycott, read The Back Story