Sometimes people cite polygamy as evidence that the definition of marriage has changed over time, proving it to be malleable according to the culture.
But even in the case of polygamy, has there really been a change in the definition of marriage?
While I could see someone today wanting to define marriage as one man and multiple women, I don't think that's how people viewed it in the past. That is, they didn't see their situation as one big marriage where everybody was married to everybody else. Rather, marriage was still simply one man and one woman. It's just that the man was allowed to have more than one marriage.
We haven't changed the definition of marriage, we've only limited the number of concurrent marriages a person can have.