Well, first of all...because they don't believe in it, there isn't such a thing. A marriage is a tenet of religion, a union of one man and one women. The RATs always screaming "seperation of church and state"....well, the government can have no standing on Holy Matrimony.
Secondly, because this is America, where we have freedom of association AND freedom of religion. The Constitution tells the government what it can and can't do...not the people.
Regarding business and "public accomodation" the federal government has no right to tell the private sector what it can and can't do.
In theory....
In point of fact, the media and the courts have seized those powers long ago and the gummint has just got into the act, mostly so they can grab mo' money and power.