This was John MacArthur’s answer after the California court ruling last year (which was later amended by California voters), I think it still applies after the ruling in Iowa today regarding gay marriage:

As you interact with homosexuals and their sympathizers, you must affirm the Bible’s condemnation. You are not trying to bring damnation on the head of homosexuals, you are trying to bring conviction so that they can turn from that sin and embrace the only hope of salvation for all of us sinners—and that’s through faith in the Lord Jesus Christ. Homosexuals need salvation. They don’t need healing—homosexuality is not a disease. They don’t need therapy—homosexuality is not a psychological condition. Homosexuals need forgiveness, because homosexuality is a sin. …

What should be your response to the homosexual agenda? Make it a biblical response—confront it with the truth of Scripture that condemns homosexuality and promises eternal damnation for all who practice it. What should be your response to the homosexual? Make it a gospel response—confront him with the truth of Scripture that condemns him as a sinner, and point him to the hope of salvation through repentance and faith in Jesus Christ. Stay faithful to the Lord as you respond to homosexuality by honoring His Word, and leave the results to Him.