Well, members of any faith, Christian or otherwise, have the right to speak out on the issues. The problem lies in the fact of what they do in general, which includes this Mormon issue that is the topic of this thread. It's not enough for them to dislike gay marriage, they have to make sure nobody can practice it.
In pushing your theology in this manner, you are effectively trying to make everyone follow the ethics of your church, no matter if they're a member or not; this is something that should elicit a major, hard-core WTF from anyone with an ounce of sense and tolerance in them.
Regardless of what they might believe, this is religion stepping way out of bounds, and at least in my view, it's grounds to fight on any level, even resorting to violence if all other options are expired.