I've read it and heard it for almost forever, and have no idea what kind of work is involved. No one has ever told me, at least I don't remember anyone sharing that information.
Is it manual labor? Or, the thinking, planning kind? Or does one just show up, punch in, and then hang around in case there is some kind of work today?
Who would want to sign on to a job involving work, when you thought it was going to be pleasure, and sharing, and cuddling, and doing things together, and it might involve children, and probably in-laws.
And who would want to take a job involving mysterious work that is not revealed, in fact it looks very much like marriage taking work is like being shanghaied on a nice walk on the dock and waking up to find one is a cook's assistant on a cargo ship.
Does marriage take work, seriously, does it? I don't think so. In fact, marriage is one of those nominalizations that doesn't exist in reality. We marry legally by signing a document, but the document doesn't indicate what is involved in the deal. We find out later that it means we're giving up a great many rights we had before we signed, and getting some vague and unforeseeable ones.
Marriage is a legal agreement, but that does not indicate work will be forthcoming.
I think marriage is whatever you want it to be, together. Maybe a little bit of work there, but not really enough to fuss about.
I also think that saying marriage takes work is sort of an excuse for dismissing puzzlements, questions, complaints, and important issues of disagreement that they don't have a real answer for, or are afraid to tell you what they really think about your own behavior or partner choice.
If you have a clue about what kind of real work might be involved in marriage, please share it in a comment.