If, upon moral grounds, you don't like the law, you can vote not guilty. I can't dictate moral beliefs, and neither can the judge. However, if asked what your thoughts are regarding the charge, you willingness to answer truthfully will most likely get you kicked off. Not always, as I got to sit on a three strike which was 6-6 going in after a half day's spent on prosecution by the ADA..
Again, you are supposed to decide the truthfulness of the testimony, and what the facts are. If you believe someone is not being truthful, it would be much easier to persuade fellow jurors not to convict, rather than try to nullify the law.