While religion is, not only a protected class, it is also an enumerated right in the Constitution so it brings with that, more protections and issues.
With the marriage and Kentucky situation you also have other things. I’m not at all familiar with Kentucky but many states and the Federal government will not hold themselves to the same standard as private business. For instance, if you’re hourly an employer can’t make you take comp time in lieu of overtime, time and half. The federal government conveniently exempts itself from this.
Add to this, the Kentucky woman is an elected official not a mere employee. This complicated a lot of things.
For private business, the EEOC says that if a person requests an accommodation or the employer should reasonably see they need one, the employer must make it.
Customer preference is never a reason for discrimination of a protected class.
Religion is a protected class, but the EEOC extends this to any system of beliefs, so in theory you could have your own personal religion but that would be harder to prove, if needed than say a Catholic, Jew or Muslim.
It does not matter when or how often an employee changes their religion so long as the beliefs of said employee are sincere.
The employer must accommodate the request, if such request does not impose an undue burdon the employer.
Here’s some examples: Joan becomes an Orthodox Jew and cannot work from sundown on Friday to sundown on Saturday. She switches shifts with coworkers. The employer must allow this.
Now should Joan not be able to find someone willing to switch the company must move her or reschedule others IF, it does not prove an due hardship. The EEOC has ruled, in cases like this, having to hire someone new or force someone to unwillingly switch shifts IS an undue hardship.
The EEOC imposes minimum standards for “undue hardship,” while other resources like the ADA impose the maximum standards for “undue hardship.”
In the airline case, it would seem that allowing the Muslim flight attendant to refuse to serve alcohol is reasonable as long as there is another flight attendant to serve it. Of course only a judge or arbitrator can decide for sure.
Remember the above is a general overview, your state may have different laws which provide one with different recourses as well as a CBA or legally enforceable employment contract.