"Calling a legal same-sex union "marriage" enhances the union by granting it the same status that straight people can have."

One interesting thing is that as soon a the Government here in the UK said they were going to introduce Civil Registration for single-sex partnerships, there were protests that since this wouldn't be open to mixed sex partnerships, it discriminated against these.

In other words for some people the idea of a Civil Registration which was not "marriage" was seen as a more attractive option.