Marriage is really important to people. If you define the legal part of that as "civil union", people will add new cultural/religious significance to that new definition. Then when polygamy or whatever becomes popular in 2115, the conservatives of that time will cry out about the "sanctity of only two people in civil union, the way it's always been!" And around the circle we go.

What's important today is that our country has expanded our liberties and rights, and a lot of people can now live fuller and more secure lives as a result.