Re: if Chakotay doesn't have a last name, when he gets married...
Of course why does any name have to be taken, Chakotay had one name. His wife depending on who would have one name or two or three. His children would have as many names as they cared to give them. Maybe the wife has a surname and the children take his custom and don't have surnames either.
I am also reasonably sure in the future people will have some official designation like a social security number so they really won't need last names.
"My name is Matai Shang and I do not exist, indeed I work very hard at it."
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