Isn't this the same basic argument that goes "anybody can get married, so long as you marry somebody of the opposite sex"?
No, because no specific individual is given a right to polygamy over another.
The argument from your quote clearly discriminated against those who wanted to marry the same gender. But in the case of polygamy, no one can be in a marriage of more than two - be they straight or gay, man or woman. It's precicesly because polygamy is outlawed for everyone that it doesn't discriminate against anyone.