Let’s put away this whole polygamy canard once and for all.
First of all, polygamy and promiscuity are not the same thing. Sanctioning polygamy is not the same as giving men license to sleep with whomever they want. Second, no one is knocking down the doors of courthouses demanding plural marriage. Why? Because women have no reason to want to be in polygamous marriages anymore. We don’t need it to bolster birthrates, and women have much better options for societal advancement than entering into plural marriages with higher class men. Men, women, and the rest of society receive no discernible benefit from plural marriage. That’s why we not only don’t allow it, but why it isn’t even an issue. Gay couples, on the other hand, have a lot to gain from having their unions recognized by the state, and so does the rest of society.
If there were people in our society demanding the legitimization of polygamy, we would have to weigh the pros and cons in a serious way, just as the best of us are doing with gay marriage, rather than simply ruling, “Well, we haven’t historically allowed it, therefore we shouldn’t,” Or crying, “If we allow polygamy, where will it end?!” Many things have been called marriage throughout human history (more than you would probably think), and the legitimization of any one of those things by modern society does not automatically lead to the legitimization of the others, and if we really believe that certain kinds of relationships should not be accorded the benefits of marriage, then we shouldn’t be afraid to detail exactly the reasons why rather than appealing to tradition or resorting to slippery slope arguments.
Maybe the language of discrimination isn’t right. Not every relationship deserves to be called marriage by the government. But what if gay marriage does? If we have good reasons to legitimize it and no good reasons not to, then we are being unfair, and unjust.