Definitional Dissonance

In his "statement on marriage equality" Senator Hoyer of Maryland wrote:

Because I believe that equal treatment is a central tenet of our nation, I believe that extending the definition of marriage to committed relationships between two people, irrespective of their sex, is the right thing to do and will not, in any way, undermine the institution of marriage so important to our society nor impose a threat to any individual marriage.

How could a new definition not change an institution based on the previous definition, because a new institution is to emerge from the new definition? Are they two separate institutions? Or one and the same? But they cannot be the latter, as one essential present in the original institution, sexual intercourse, cannot be present in the new definition and institution.