There’s been plenty of praise and attention for the two young students who have filed a ballot initiative that would remove the word marriage from state law and replace it with domestic partnership. In other words, the state would be out of the marriage business. Churches could still bless marriages. But that’s it. The fact that so many liberals have embraced this idea demonstrates how little the left understands about same-sex marriage and marriage itself.

Getting the state out of the marriage business is a truly terrible idea in every way. Worst of all, it will cause great harm to the cause it’s ostensibly supposed to support: marriage equality. In fact, until I read the news coverage showing that the sponsors were sincere, I had suspect that they were secret opponents of same-sex marriage who want to sabotage the effort to repeal Prop 8.

If you’re a liberal who can’t see beyond the facile argument that “the law should be equal, you might not understand how this initiative plays into the hands of same-sex marriage opponents. How’s that? Those opponents are always at pains to try to describe how expanding the right to marry to consenting adult couples is a threat to marriage.

That’s a hard argument to make because same-sex marriage isn’t a threat to anybody’s marriage. Simply put, the same marriage laws that apply to opposite-sex couples should be extended to same-sex couples. End of story.

But the supporters of this initiative– and the idea of taking the state out of the marriage business – are in fact threatening the marriage of anyone who wants to get married – but doesn’t want to do so in church. Conservatives opponents of same-sex marriage are already using the initiative as an organizing tool. Look, these folks say. The same-sex marriage people want to take away the right to marry.

In fact, the marriages threatened by this idea are those of couples who, polls show, are most supportive of same-sex marriage: secular straight people. My wife and I probably fit that demographic. And I have to say that if this initiative were to become law, it would turn me from a strong supporter of same-sex marriage into a reluctant opponent of same-sex marriage. I’m happy to bring same-sex couples into the fold, but not if it means that couples like my wife and me can’t get married. I suspect I’m not the only member of a straight, secular married couple that would feel this way. And if the marriage equality cause loses secular straights, Prop 8 will never be repealed.

Same-sex marriage supporters need to recognize this initiative as the threat it is and quickly and emphatically come out in opposition. Same-sex marriage supporters need to make clear that they don’t want to end marriage or change it in anyway. They simply want to welcome committed straight-sex couples into the club.

That’s the right approach politically. It’s also the right approach as a policy matter. Perhaps young college students are too young to understand this. But while conservatives are wrong to oppose same-sex marriage, much of what folks on the right say about marriage is correct. Marriage is a foundation of our society—our civil society no less than our religious society. Marriage as an institution is crucial to the health of communities and society. Children do best when they’re raised by two people in a healthy marriage. Marriage is not just a religious institution: it’s both secular and sacred

In fact, the reason why same-sex marriage should be permitted and encouraged is because marriage is so important to our society’s health. We can’t afford to lock responsible adults out of such an important institution. To the contrary, we need to bring people, regardless of sexual orientation, into the institution. This foolish initiative does the exact opposite.