I'm not sure all this quibbling about what counts as redefinition is really important. The legal definition of marriage has been changed by allowing same sex marriage but the nature of marriage is totally unaffected neither are any of the legal rights or obligations for people already married.
As you have pointed out, this change is insignificant compared to some of the previous changes is the "definition" of marriage. In the Hebrew Bible, it was an institution between one man and at least one woman, sometimes hundreds as pointed out by LR. In more recent times, marriage has been used as a tool for diplomacy and business. Love didn't come into it for the ruling classes. It's quite recent that marriage didn't effectively make the woman a possession of the man.
The definition of marriage has change continuously throughout history. It's nonsense to suggest that we can't make this little change to extend it to more people just because it's never been done that way before.
Marriage is a constantly evolving social construct - there is no doubt of that.
Regarding definitions - I think it depends on whether you include those who can, and cannot avail themselves of marriage, within the definition or whether that is effectivity an eligibility element.
If you include the eligibility criteria you end up with a horrible clunky definition, even now when eligibility has been extended for civil marriage. So you'd end up with:
'A consensual loving and legally-binding commitment made in public between two people of sound mind who have not previously been married and/or have completed the process of divorce, who are over 18 (or 16-18 with additional parental consent) and who are not close blood relatives or are an adoptive parent or child of their intended spouse or are an in-law depending on the circumstances.'
And this would have been even more clunky in the past, and/or in religious of other jurisdiction context, when there were restrictions on same sex marriage, divorcees being able to marry, marriage between people of different races, restrictions on marriage between people of different religious faiths etc etc.
Sounds a bit clunky to me - in my mind better to separate the heart of what we consider marriage to be currently (what marriage is):
'A consensual loving and legally-binding commitment made in public between two people'
From the eligibility criteria (who can and cannot get married)