Incoherent?

I don't really follow this whole debate...

From Ben Smith:

Obama spokesman Ben LaBolt said: “Sen. Obama supports civil unions, and he has consistently opposed federal and state constitutional marriage amendments because as we have seen in some states, enshrining a definition of marriage into the constitution can allow states to roll back the civil rights and benefits that are provided in domestic partnerships and civil unions."

So Obama opposes the amendment (despite also opposing gay marriage "at this time") because it might lead to rollback of even civil union privileges, which he believes are non-negotiable. Where's the contradiction?