In reality the "gay marriage will lead to polygamy" argument is humorous because the opposite is true. Polgyamy has lead to same sex marriage. Polygamy existed before same sex marriage, was outlawed before same sex marriage, and is the first argument brought up when considering same sex marriage. People have effectively used the banning of polygamy as the defining factor for two person marriages. However, the flaw in that definition is that it doesn't justify restricting the genders of the two people getting married to being different. So people have no put the wagon in front of the horse and now argue that what they have used to define marriage, the banning of polygamy, will fall apart if two people of the same sex are allowed to marry. It doesn't make a lot of sense, but meh.