The problem with this, as mentioned above, is that when DOMA goes away, there will still be no federal benefits for civil unions, nor a large likelyhood of identical benefits being established. Even if you do establish identical benefits, then you have a situation where the only difference is in name, which makes two sets of identical laws kinda stupid.
You say "when" DOMA goes away. That's not altogether certain. Regardless, if must first go away for remaining dominos to fall. You mentioned 5 years in a previous post. A lot can happen in five years .. and it may take longer considering it was passed by a huge margin in both the House and Senate. Anticipation may be personally specific, not at all manifestly guaranteed.
If there are currently no federal benefits for civil unions, Prop 8 supporters are asking why can't there be -- why is it unlikely that identical legal benefits be established. I'm sure they know full well that without such legal protections being made at all levels of law for civil unions, they are likely to lose their approach. Getting equal protection applied to civil unions is pivotal for Prop 8 supporters.
It seems the Prop 8 suggested approach was to secure what has long been their traditional definition of the name of the cultural institution that is greatly valuable to them as they perceive it ..
.. And then the mandate will be there to correct all level of law to support equal protection for civil union couples.
The Prop 8 supporters do not think that would at all be stupid to have equal protection under the law for both marriages and civil unions.
What they find egregious is to have the cultural institution that has long been meaningful to them as defined to be redefined to mean something else shorn of its meaning.
We may not all agree with them, but their point of nomenclature is valid and significant.
They don't mind that cat owners have cat shows with the same rules for entry and contest. What the Prop 8 dog show people are objecting to is being forced to let cat owners enter cats in their dog shows .. as those events are then no longer dog shows, about which they are most certainly accurately aware.
Naming something to reflect what it truly is a foundational way of truly understanding what that entity means -- between a man and a woman as husband and wife -- which they see as what marriage has long been, both to them, and to our long-established culture.