There is already a Federal definition of marriage, and changing it requires legislation. Pres. Obama cannot do it by executive fiat. This definition is found in Title 1 of the US Code, Chapter 1, Section 7, and reads as follows:

In determining the meaning of any Act of Congress, or of any ruling, regulation, or interpretation of the various administrative bureaus and agencies of the United States, the word "marriage" means only a legal union between one man and one woman as husband and wife, and the word "spouse" refers only to a person of the opposite sex who is a husband or a wife.

The laws governing Federal benefits available to veterans (and their survivors), Social Security recipients (and their survivors) and Federal employees (including health and life insurance and survivors annuities) are all governed by this single paragraph in the Code. Regardless of the existence of a Democratic majority in Congress, it is questionable if he can muster sufficient votes to change this.