I think the term you're looking for is "separate but equal" and these here United States done did tried that with them thar nigras. The courts ended up deciding that was stupid and racist (much like the previous sentence itself).
In any case, marriages aren't the same as civil unions, mainly because one state doesn't have to respect the civil union of another, and there are various issues dealing mainly with tax benefits and whatnot.
The only reason I've heard that I respect for maintaining a difference of rights of gay and straight couples is the amount of lawsuits and revoked tax privileges against churches who want not to marry gay couples because the union is against their belief system. (This is not to say I'm not for gay marriage, this is just to say stupid people and stupid religions still exist, and last I checked, freedom of religion was still pretty important to us, no matter how stupid said religion may be)