This isn't about discrimination, it's about the right to choose the people you do business with. I'm on the seaside right now, and I see many shops with signs indicating they don't serve customers without a shirt or shoes. It's also forbidden to sell alcohol to minors, but nobody sees this as discrimination.
Thats exactly correct. But the govt cannot use that to stir up division within the people.
It's also about the erosion of your freedoms. You no longer have the right to choose to get healthcare coverage, if they have their way you will lose your right to own a firearm, you right to free speech is being restricted everyday and your right to even have a political opinion is going away (see the CEO of Mozilla who was forced to resign for a measly $1000 contribution).
This falls into that same boat, your right to run YOUR OWN BUSINESS the way you want to, is going away. This is a sad story for America, no matter what your stance on marriage is.