Your question reduces to "Why should my taxes pay for a public good, when I choose to have my kids not experience it directly?"

I think the answer becomes a lot clearer then.

Miss Manners has a quote something like "School taxes are what we pay to the future in hopes that it has things like nice young geriatric doctors in it."

Also, what precisely are you claiming is "secular indoctrination"? Evolution? The separation of church and state (in the US)? People who went to religious school have confidently told me that both are myths. They also told me that their school absolutely should have gotten tax funding.