He's right about this. No Constitution or any other human law can give a right to sodomy, a right to abortion, or a right to otherwise violate the law of God. Such a law would be a nullity. There are certain things that can never be amended.
Brown's problem is that he is holding sodomy, and worse--societal recognition of sodomy--as the "right" that trumps the Constitution. It is the reverse of the natural order. He wants all things subservient to a sin, whereas all things are subservient to God. If this argument holds up, California will be part of a nation under sin rather than a nation under God.
That’s true as well: no constitution may trump the natural order.
If Brown framed his argument as an argument from natural order, given that the Constitution allows for amendments, and he already admitted that this is a validly executed amendment, he would have to say that he believes the California constitution is itself in violation of the natural order. Mr. Brown can start making this argument by resigning the office of Attorney General, and proceed as a spiritual leader that he has positioned himself to be.