To hear some folks speak, same sex marriage is worse than the genocide in Darfur, the occupation of Tibet, or than the systematic torture and miscellaneous violation of human rights conducted at Guantanamo.
Iowa's Supreme Court recently ruled that the benefits of marriage must be open to all people. Initiatives are underway in other U.S. state legislatures to enable same-sex couples to get married.
As a result, the National Organization for Marriage is organizing against them. I've linked to their talking points.
Most of their talking points are laughable and seem motivated by hysteria. Religious groups would not lose their tax exemptions. While it's true that schools would eventually teach kids in sex ed that some couples are same-sex, they would not "stigmatize" the idea of opposite-sex marriage.
One of their talking points is very worrisome. In Michigan's 2004 election, many proponents of the state's marriage amendment said that they would not seek to overturn arrangements allowing domestic partner benefits. After the marriage amendment passed, some people, such as the state's attorney general, successfully prohibited public universities from offering such benefits (see note below).
It may be that most people who voted for the amendment would not wish that to happen. The fact is that there is still significant discrimination on the ground. There are people in high places who are willing and able to do evil.
Additionally, many employers in the US are not willing to grant domestic partner benefits. Without formally recognizing marriage arrangements between same-sex couples, it may take considerable time for them to do so.
Lastly and most importantly, allowing same-sex couples to marry is only a threat to the really narrow-minded.
Edit: I was informed by an attempted commenter that the U of Michigan went through some legal maneuvers to retain its domestic partner benefits. This is correct.
The fact remains, though, that some conservatives attempted to stop public entities in the state from granting such benefits. My anonymous commenter did not admit this, so I will not publish his or her comment.