There is no general prohibition against "discrimination" (a process that, broadly understood, is quite proper and necessary) or even against irrational or unfair discrimination. The Civil Rights Act of 1964 prohibited only certain specified forms of discrimination. The major categories are race, religion, and ethnic origin. The provision concerning sex discrimination applies only to employment, not to public accommodations like restaurants. As a result, for example, McSorley's Old Ale House in New York City famously did not admit women until 1970, when New York City law compelled it to.
Other forms of discrimination weren't addressed at all. That's why, for example, there was a later Age Discrimination in Employment Act.
Nothing in the Civil Rights Act prohibited discrimination based on sexual or affectional preference. That's why there was a push, similar to the push for the ADEA, to pass a new bill, the Employment Non-Discrimination Act, that would ban such discrimination in employment. It passed the Senate late last year but is expected to go nowhere in the House. Even if it passes in its most inclusive form, it wouldn't apply to places of public accommodation. This bigot in Oklahoma would have to hire LGBT people as servers but could still refuse LGBT customers.
Note that some states and localities have protections that are broader than those of federal law. A state or locality can't authorize discrimination that's illegal under federal law, but it can go beyond federal law in ensuring equal treatment. It's safe to assume that neither Enid nor Oklahoma is in the forefront of such efforts.