I would argue that for the same reason the symbolism of the word is important to homosexual couples it is important to heterosexual couples. Why force one group to alter that to appease another group? And why is that the role of the government?
It is not. This is why I think the government should back away from both marriages and civil unions. If one person wants to change their last name, their PNoK, authorize a general PoA, etc., then let them. Do not confine this to any type of social institution though and do not make the process more or less stringent dependent upon the individuals involved.