I think that both marriage and civil union should be options. If a gay couple feels that marriage does not meet their definition of their union, then the option of a civil union is there. Instead of trying to change the concept of marriage (i.e., the terminology, religious connotations, etc.), they can choose to be civilly united instead. In this way, the "sanctity" (I hate that word - LOL) is spared for those who feel it is important, and gay people (and straight people) can choose which union is better suited for them.
IOW, I think both should be offered, and let the couple choose, but if they choose "marriage" then they are choosing all that goes along with it, the terminologies, etc.