Bull. Words evolve constantly, in every language. There are even some words and phrases in English that have evolved so much over time that their current meaning is the opposite of their original meaning. You certainly can change the definition written in a dictionary.
In my view, it's not the word that has changed. Rather, our understanding of what constitutes marriage has gradually changed over the centuries. At one time, a marriage was the ownership of a woman by a man. A woman could be fired from her job if she got married. She couldn't vote because she was not believed to have her own mind independent of her husband or father.
I'm talking about fewer than 100 years ago in the US.