Allow me to riddle you with topical bullets.
I agree with David French, the conservative religious liberty commentator who has recently earned some criticism, that civil marriage is a different animal than Christian marriage. The civil kind was invented by Napoleon to advance a secularist authoritarian state, and to that effect I wouldn't mind if it was abolished (along with many other machinations of the secular nation state).
Civil marriage is, therefore, a tool. It confers an economic incentive structure, which the state can use either to good or bad ends. Ideally, this tool encourages people to marry, build families, and continue in the kinds of ordinary life that are essential to the good of the commonwealth.
However, since the goal of this institution is the good of the people, it should not be used to incentivize detrimental behavior. If homosexual relationships are detrimental, then the incentive should not include them. And are they detrimental? The secular world says no, obviously, but with Christians the response varies. Some Christians will say that homosexuality is detrimental only to those engaging in it, and others will say that it is detrimental to everyone. I would argue that the latter view is far more orthodox: we are communal creatures and our actions can't ever be done in true isolation. "I the Lord your God am a jealous God, visiting the iniquity of the fathers on the children to the third and the fourth generation of those who hate me."
So, I disagree with the popular libertarian sentiment that as long as a behavior doesn't hurt anyone else, it should not be stopped by force - because all behaviors affect everyone. That said, I don't like force, and force isn't part of the discussion. The discussion is about rewards given to degenerate behavior. I don't like big bureaucracies for the same reason: they reward liars and manipulators. Admittedly, it's hard for me to equate those groups with active homosexuals, but scripture is clear and I think my bias has more to do with cultural conditioning than with logic, instinct, or theology. It's not only Christians who have disapproved of homosexual behaviors; even many secular and pagan historians connected those sorts of behaviors with the falls of the Greek and Roman civilizations. Our culture is more of the exception than the rule.
So why push our own vision of what is beneficial? After all, there are many people in the world with many viewpoints. Why not keep a neutral public sphere? Because the concept of the neutral secular government is a farce and one of the best weapons used against evangelicals to avoid supporting their own interests and beliefs. Those in power always push what they believe to be beneficial, with effectively zero exceptions. Secularism as it exists in the real world is rife with assumptions, values, and agendas. Why let these people push their vision of the good life when we know it to be wrong? We need to be careful to avoid being moral relativists.
Policy has nothing to do with the gospel, and I think David French would agree with that statement. Therefore, policy cannot be the mechanism by which we witness to others. It can only be the mechanism by which we love others, by incentivizing the things that do everyone the most good.