I live in California...so it's valid here.
Well still, your point is that unless you have a binding legal contract with the person you have sex with then you're committing adultery? Does that mean for the thousands of years before the idea of marriage popped up that everyone was cheating on everyone?
EDIT: plus, where I live it isn't legal to get married. So for me you have no point. So with your logic I'm not committing adultery. Why do I get to be an exception and everyone else is a sinner?