While I actually support that move, there are a couple of problems with it.
1) Separate is inherently unequal; what this means is that in order to actually make a valid domestic union relationship, you'd need to actually abolish the current relationship of marriage (i.e. so everything that was a marriage is now a domestic union). This would be hard to get the civilian masses on board with.
2) Following on from the above: you can't retroactively remove the marriage that people already have. Even if you could, creating a new relationship and abolishing the recognition of marriage would create a mass of administration costs of people needing to reregister their marriage.
3) Marriage is actually not a religious term inherently; the legal definition exists without the religious connotations (consider: multiple religions have slightly different versions of marriage, hence there's no religious holistic definition). There are arguments that the religious lobby should not be able to castigate the term for themselves and deny it to the general populace.
1. They are not separate. And they can easily be made equal by simply making all unions a "Domestic Partnership" in terms of the State, it's just people who get married in a Church can have their ceremony be called a "Marriage". This has nothing to do with the "separate but equal" ruling because it is quite easy to give everyone the exact same rights. The only difference is religions can hold onto their word "Marriage".
2. I never suggested that, I suggested rephrasing it to a "Domestic Partnership". I fail to see how you could have drawn this conclusion from anything I said.
3. The concept of Marriage has been a religious one for far longer than it has been a legal one, and given this, the religious lobby has "first dibs" on it, in a sense. The religious lobby has every right to deny the "general populace" (which isn't the case by the way-far more than half of the United States is religious) the use of the term in legal definitions because
they got to it first. The word is their intellectual property. Besides, why does anyone who is not religious care about the use of the word? They only reason anyone brings it up is to upset the religious lobby because they hate religious people and like to see them upset for what they consider to be silly or stupid reasons.