Proof is unnecessary.
If you tell us you are married we will consider you married unless we have information to the contrary. We will also consider you married, on the basis of your statement, if you say you are living with an unrelated person of the opposite sex and you both lead people to believe you are married. If you are a child to whom the parental deeming rules apply and we receive information from you or others that you are married, we will ask for evidence of your marriage. If you cannot, you must tell us why not and give us whatever evidence you can.