Because there are many people that won't be happy.
There is a case passing through the Supreme Court right now that will decide if marriage licenses issued to same-sex couples can be permitted by states. This became an issue as some states where same-sex marriage is not permitted have laws that require them to accept marriage licenses issued in other states, giving a work around to same-sex couples.
So, what will happen if it passes, and states are all told they are allowed to make this a law? While every state would not change immediately, if the licenses must be considered valid by other states, then same sex marriage will be on the road to complete legalization.
Then, what happens? Christian leaders in both Congress and outside the political sphere have spoke up to say this: we will not follow the Supreme Court's ruling if that happens. They have said they will not marry LGBT couples.
But that's where I don't get where their paranoia is coming from: no one is asking them to marry these couples. Nobody is going to make a church marry a LGBT couple if they don't want to. It's a legal definition that they're going for. Not trying to convince the Pope to change some major doctrines!
Personally, I'm really excited and hopeful that the Supreme Court's decision will put us on the road to a more equal marriage environment here in the US.
What will happen if same-sex marriage is essentially legalized?