Immigrants are just individuals? Not married or unmarried?
Of course they're married or unmarried. But there doesn't need to be a distinction between married and unmarried immigrants under Federal law ... any more than there needs to be a distinction between a 26 year-old immigrant and a 27 year-old immigrant.
The word "marriage" is not found anywhere in the U.S. Constitution, which is itself a compelling argument against any distinction between married and unmarried persons under any Federal statute.
You think that the military is just something that you wait for a war, and then you go make some people Admirals and ship captains, and build some ships and jets, and submarines, and then start training Aircraft carrier drivers and submarine crews and building radar stations and so on?
Do you know that the America’s government was defining marriage for it’s troops by at least 1780?