Why didn't Protestants argue for traditional marriage by natural reason? The US Supreme Court recently ruled that Homosexual couples can get married in all US States and that these marriages are legally equal to the Traditional model of marriage, one man and one woman. Prior to all this I saw many Christians arguing against the redefinition of marriage, but almost solely used their faith and the Bible as their basis. 

There are arguments against the idea of homosexual marriages being not equal to heterosexual ones. They more or less revolve around the idea that the reproductive system is the only part of the human body that needs a complementary body to unite with to complete and achieve its end (or purpose). Any other union that cannot complete the system and therefore possibly achieve the purpose of sex is inherently incomplete.

My question is this: Why didn't Christians argue this way? I am Catholic and I have seen my own church reason this way. Why did Protestants not use this, as this could carry weight in a secular court?
"Protestant" means a believer in sola fide and sola Scriptura. As such, not only is it possible for a Protestant to use natural law arguments, many have. Although most of the reformers were Divine Command theorists (Luther and Calvin, although Calvin is weird), not all were (natural law was influential among both the Catholic and Protestant American Founders, although with a clear Lockean (Divine Command) influence as well). Many Protestants today are natural law theorists, although not all are.

For information on the Founding Fathers: http://scholarship.law.nd.edu/cgi/viewcontent.cgi?filename=5&amp;article=1001&amp;context=naturallaw_proceedings&amp;type=additional

For an article on Protestants and Catholics regarding Natural Law: http://www.firstthings.com/article/1992/01/002-protestants-and-natural-law

Christi pax.