Perhaps there are other organizations in your child's school that do much good work but also support events or causes that you cannot contribute to in good conscience. Here's an idea from Deerfield mother, Laurie Higgins:
The pocketbook and loud public dissent are the only forces that may effect change, and loud public dissent is rare. We seem to be a bunch of silent sadsacks. So, let's speak with our money. It's less scary.
Write a letter explaining why you are withholding their charitable donation and send it to the organization's leaders. If you're brave, you could also send a letter to your local press. If you feel so inclined, you can contribute those monies to support something else like the Athletic Booster Club. If you do, you should make that clear in your letter. Finally, encourage like-minded friends to do likewise.