I'm not signing it. In my humble opinion, Christ's mission for believers is not primarily to champion traditional marriage, it is to make disciples from out of all nations. The MD supplants what is most important with what is kind of important.
It does not preclude a Christian from spreading the gospel. Christians can walk and chew gum at the same time. It certainly is a timely and serious issue.
There is a proper order of religious obligations in this life, and the worship of God must always be #1. Every other good thing is subordinate to that, but flows from it and is harmonious with it.
Action for justice in culture, in custom, and in law is a constitutive element of the preaching of the Gospel.
There could be a difference between “as a Christian, I say...” and simply “I say.” Speaking as Christians we ought to echo the words of Christ faithfully. But we can advocate personally for any number of good and right causes, from humane treatment of animals to better sanitation to good social family values.