An African-American friend just shared that a church-going Facebook friend of hers was critical of her public stance in favor of marriage equality. My friend argued back in terms of the history of discrimination against their people, and that's a powerful comparison.
I did something similar at a talk I gave in a Michigan synagogue some years ago, raising homophobia and anti-Semitism, but I went further when one man quoted Leviticus 18:22. I asked him why this particular prohibition was so important to him. He couldn't answer, in the same way that opponents of marriage cannot explain how it "undermines traditional marriage."
I also asked which meant more to him, a prohibition like that one in Leviticus or the much more life-affirming and transcendent Golden Rule in Leviticus 19:18.
I agree 100 percent with everyone who believes that religion and secular law shouldn't mix, and that invoking the Bible against any legislation is simply invalid in our secular country which does not have an established church. But I also don't see why proponents of marriage equality should cede ground when it comes to persuading people who quote the Bible as if that trumps every argument. Why not meet them on their own turf?
If a Christian friend starts quoting the Bible, ask why same-sex behavior isn't worthy of being listed in the Ten Commandments. Ask why Jesus doesn't mention it. Ever. Ask why he brought so many marginalized people to his table, people like reviled tax collectors and prostitutes. And ask who's more like those folks in our culture right now than gays and lesbians. Who knows, you might change some minds.