If that clause was in his contract I'd agree with you. I admire standing on principle. What I don't admire is not fulfilling your duties in the job you were hired to do. Again if your personal convictions were so strong you owe it to your employer to inform him before you accept employment. Otherwise don't cry when he cans your ass for not doing the job he hired you to do.
That has already been addressed by others above. Nice try, doesn’t wash.