Wrong on both counts. For much if not most of history, marriage was one man and a bunch of women, or even one woman and several men.
And it is NOW frequently one man and another man, or one woman and another woman. Thousands and thousands of same sex couples are now married, and always will be.
This idea that families have always looked exactly the same is absolute bunk. The idea of what is a family has evolved a great deal over time, and this change to recognize the gay people who have ALWAYS existed but previously lived a miserable tortured life of hiding and lies and can now live openly as who they are is a very good thing for all concerned and harms absolutely nobody.
Most often by far, marriage has been one man and one woman. Only the wealthy or rulers could afford additional wives. While there may have been a wife with several husbands, it is so rare, I can't think of one historical example.
Moreover, the issue is not 'if it harms anyone', it is are they the same relationship? Obviously 'no' on every level of observation.
Words define reality. When we verbally distort reality with words, there are always negative consequences. Psychology terms it denial.