Straw horse argument. It's not about a particular marriage; it's the institution of marriage which we don't want to be forced to redefine.
A counter question would be, what should the definition of marriage be, and who should decide what that definition is?
I’m definitely with you.
Either marriage is defined objectively as it is in the Bible as one man and one woman,
or there is no objective definition, and whatever anyone of any generation wants it to be, that is what it will be.
There is no way that it will go anywhere but in the direction of
any number of men, their daughters, their pets and the pets’ fleas constituting a “marriage”.
And if you change the definition of marriage once, how many more times will it be changed?