I agree, except that I think it’s very little (a) and very much (b).
This amendment simply enshrines into the constitution what is already law in Virginia. It will be that much more difficult to remove it later. You should vote for this amendment only if you believe that same-sex couples should be permitted neither marriage nor civil union status in Virginia. It’s unclear what, apart from simple animus towards gays, could motivate such a view. If you believe in limited government, for example – a traditional conservative view – surely you would support the right of two people to contract, between themselves, an arrangement that would approximate marriage.
But as you correctly point out, among the many correct arguments against this amendment we do NOT find “it will destroy domestic violence protections for unmarried couples.” That’s not so, and I find it very disturbing that opponents of the amendment feel they have to concoct specious arguments to defeat it.