It is occasionally argued that since some of the arguments against legal recognition of polyamory sound like some of the arguments raised against same-sex marriage, and since same-sex marriage is OK, then polyamory must be OK, as well.
The arguments that were used by the GOP to argue for the impeachment of President Clinton sound like many of the arguments used today by Democrats for the impeachment of President George W. Bush. Does that mean that Bush and Clinton are the same?
Extending equal marriage rights to same-sex couples is not the same thing as extending marriage rights to groups of three or more. They are, at least, different enough to require some careful analysis and consideration.