"Marriage is an institution with the primary purpose of providing a good environment for children and their biological parents. We only permit sterile straight couples to get married to promote the illusion that marriage is about love and happiness for the married couple. Since a polygamous household is not a good environment for children, we ban polygamy."
Continuing in the tradition of stating opinions that are not ours but are potentially interesting:
One could argue that at the time these laws were made, checking for sterility was impossible and is now potentially prohibitive or inaccurate as medical science improves.
The most one can do is show that some more generally accepted subjective value implies the a specific subjective value. A more general argument: "any act performed in private between consenting adults is morally acceptable. This implies gay sex is OK."
(That's also a specific example of a general argument which most proponents don't actually believe. See this comment thread for an example: http://news.ycombinator.com/item?id=806581 )