I bolded yours. Marriage insinuated the exchange of property in many cultures for much of time, but that was not the definition of marriage, that was a culture's view on the place of a woman, not marriage.
Definitions are irrelevant. It is how the systems works within a society. The definition may contradict how it is being actively used by cultures, but the actual distribution of marriage and marriage policy has changed.
Let's not forget, definitions change all the time as well.
The word "Person" has equally changed over time. It used to mean all humans, then all white male humans (in the U.S.) then all male humans. It wasn't until Southern Pacific Rail Road that it came to mean non-humans (corporations).
Definitions are also constantly added. How many definitions do we create a year?
They are amended as well.
So if neither definitions nor use of the system are constant unchanging things, why must they remain in their current iteration?
Further, even if they were constant unchanging things, why is that a reason they must remain so?