How ignorant are you?
Oh god. No they don't.
This is about gay rights.
Because this isn't protesting, and should be acceptable.
Based upon what? The assumption that all Christian centered companies hate gay marriage?
Have you ever worked in a professional office environment? As for my "source", look in any companies policies and procedures, everything is clearly laid out regarding religion or politics in the workplace.
And yes, you can get in trouble for celebrating Christmas at certain companies. At one of my last engineering jobs, Christmas was coming around, and every year we have a "Holiday Party". Well, one of the department leads decided that he wanted to make flyers promoting the "Christmas Party", he was warned to replace Christmas with Holiday because the Christian holiday could offend some people.
If you don't work first hand in these types of places, don't speak on anything.