You could argue that the right of self defense is so important that it trumps property rights. I disagree unless there is no choice but to go onto said property. If there are other places reasonably available to you that provide the same products and services then go to those places.
Using the protected class argument just doesn't make any sense. A protected class is a group of people protected against discirmination or harrasment.
Race, gender, ethnicity, veterans status, and disabilities are all examples of protected classes. Where exactly do guns fit into that?
A no guns sign is completely different than a whites only sign. One seeks to exclude a tool and the other seeks to exclude a whole race of people.
An argument touting the importance of the right of self defense over property rights is logical but the protected class argument just doesn't make any sense.