Legal Question in Family Law in California

Unfortunately yes unless you're in the military. They have adultery laws I believe.

Actually if you are in the military, you could face prosecution under Article 134, which does not specifically prohibit adultery. You can read about that section and adultery prosecutions here:

http://usmilitary.about.com/od/justicelawlegislation/a/adultery.htm

But if you are not in the military, and the other person is not, you don't have to worry about any California law.