This is a very interesting argument. Not the issue itself exactly, but the form the argument is taking.
Quite frankly, Bill is correct in his argument. What you have to remember is that he is not always talking about the legality. His main point for posting was to discuss the actual argumentation, not the merits or demerits of gay marriage, homosexual marriage, nor any other kind of marriage. Simply that the case that the argument for allowing gay marriage can't be used to justify polygomous marriages has been proven to be false.
When asked, he gives his bias. His bias is that marriage remain between one man and one woman. He justifies it based on his own perspective. What I think is important, however, is that he doesn't absolutely condemn a legal rendering of the definition of marriage to mean homosexual unions as well, and as a matter of law he understands that it's not all about his will. In fact, he has more then once reiterated that people should be able to be with whomever they choose.
Once again, though, I have to ask why polygomous marriages are taboo if one agrees to change the definition of marriage? One example used was sexism, however there is nothing that says that the marriage would have to be between only one man and multiple women. One can throw one woman and mulitple men, or multiple men and women in a single marriage.
Is it a money thing, for instance tax laws. Well, have you seen our tax laws as they currently stand? There are so many convoluted laws that if you took the same information to multiple accountants, you would get as many different returns as the number of accountants that you went to.
If it is about inheritence, then that is what wills are for. And if the will isn't satisfactory after death, that is what courts are for.
So, again, I am not seeing why an argument for polygamous marriage is "silly" if we are allowed to change the definition.