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?