If you pull up the discrimination laws (my internet connection at work is wonky - I can access here but not wikipedia or any us government site today, for some reason), there are actually around a dozen or 15 protected classes. Interestingly, they do NOT include sexual orientation or preference at this time. Each of these classes are groups that are blatantly discriminated against, and against whom the discrimination straight up would NOT stop without legal intervention.
There was no way on Earth that racial discrimination would die down in the South on its own; even today, it's still pervasive and widespread; it's just not as overt as it was. (Although you can still find it if you scratch the surface; I remember encountering a "No Niggers or Northerners" sign at a gas station I stopped at in Tennessee around ten years ago.)
It's the same thing here - these people hate gay people, don't know the first thing about them (as expressed by the belief against all fact that sexual orientation is a choice), and will continue to discriminate unless prevented. If it were a few people here or there, then there would be no need for legal protection. Unfortunately, look around at all the conflict and hatred and you can see that gay people are still very much second-class citizens who are not afforded the same rights as straight people. Hell, walk down the street holding a man's hand and watch the reactions people give you.
It would be one thing to discriminate against people for choices made - I have no problem with refusing service to a pedophile or murderer, and understand someone refusing service to a Democrat or Republican, a conservative or a liberal. It's another thing entirely to discriminate against someone for something over which they have literally zero choice.
Edit: Also, if sexual identity and orientation were a protected class, it WOULD be illegal to discriminate against LGBT people in almost any situation. Unfortunately, as I said, they're not on the national list and most states specifically exclude them from protected status.