Look, everyone will have a subjective response to this, because (a dangerous thing to say here, but I'll say it) the acceptability of the homosexual lifestyle is far from settled in this country. I'm a Christian and I can tell you that some Christians find "flaming" behavior and obvious transsexuality very unsettling. (The merits of this can and should be debated elsewhere.) You may argue that Christianity should be more accepting of "alternate lifestyles", but everybody draws the line somewhere. For many Christians, the fact that homosexuality is roundly condemned in both the Old and New Testaments, with the closest "positive" example being the open-to-interpretation tale of David and Jonathan, gives them most of the Scriptural endorsement they need for the discomfort they feel.
Thought experiment: let's say 3 known pedophiles, or 3 known animal sodomists, walked into your store and demanded service. Are you saying that you wouldn't have at least some issue with dealing with them? Because the revulsion you (probably?) feel is akin to how some Christians feel near flaming homosexuality. Unfair? Stupid? Unenlightened? Perhaps, IYO. But pretty much the way things are.
And, if you're wondering, I know and am friends with some homo/bisexuals, and I don't get in their faces or witness to them. But they know I'm uncomfortable with their lifestyles and understand. I hope the significant gay population here will take my comments above in the context of the original scenario.
Kevin
Kevin,
Consider that you and I both say we are Christians, yet my faith informs me such that I find homophobia abhorrent. I feel this way because of how it fits with what I think is the absolute core of Christianity. So its not a small thing with me. I am deeply religious, so this will not be an anti-religion rant.
Given that, you and I, both claiming to be Christians are the perfect example of why in the public sector, no special consideration should be given to a moral opinion derived from religious principles compared to opinions derived from other sources. A non-Christian is perfectly justified in looking at our polar opposite opinions and saying that moral opinions that are religiously derived are simply arbitrary, and most probably irrational.
So I have to say that given this particular work situation, religion is not a valid excuse for homophobia. Given your extended example of a known pedophile or a known animal sodomist walking into the shop, my faith informs me that I would also have to add homophobes to the list because I find them as abhorent as any of the others. See how arbitrary this exercise becomes?
What about this list: Suppose a well known white supremecist, and Pastor Fred Phelps, the well known rabidly homophobic Pastor of Westboro Baptist Church walked in. Is it ok for me to refuse them service because I abhor racism and discrimination of all types? Or should I give Pastor Phelps a pass because his homophobia is religiously motivated? If so, suppose you happen to walk in at the same time. I would have to group you in the same category as well, if I knew about your religous homophobia.
So I have to discard the religions motivation behind all this discrimination and examine it from a different point of view in order to form an opinion on the behavior of the store clerk in the OP.
From the point of view of the public sector, the American one, that is, there is a Constitution that guarantees equal rights for everyone, so we know what the law thinks about whether a gay person can make a purchase in a store or not. The bad news for all those who would want to discriminate or otherwise restrict the rights of any human being in our society, we have both the Constitution and we have hate crime laws.
In the light of civil law, then, your example breaks down because pedophilia, beastiality are both illegal in most states, but homosexuality is not prohibited in most states.
But ultimately this OP's scenario should be looked at as a work related event. Here we have an employee who refuses to perform his job function when it involves customers from some significant percent of the population (15% in Los Angeles, for example). I say this is a serious hardship for the employer, since this is a retail store and the main job of the employee is to sell merchandise to customers.
The fact that the employee claims that it is a hardship for him to do that would fall under the same guidelines as other hardships, such as a disability. In these cases, an employer is not obligated to hire someone with a disability if the disability would clearly interfere with the main function of the job.
For example, a hospital is not bound to hire a blind brain surgeon, or a comatose receptionist. However, if the disabilty would not prevent the employee from performing his main job function, the employer is expected to make reasonable accomodations. For example, a software developer in a wheelchair could surely develop software and he would be in his rights to request that the path from the car to his office be wheelchair accessible.
In summary, the employer is totally in his rights to fire the cashier for refusing to perform his job function when faced with customers of a certain demographic, be it racial, ethnic, gender, economic, or sexual preference. And an employer (or anyone else for that matter) is certainly not obligated to give any special consideration to an employees desire to discriminate for religions reasons.
Religious morality is especially suspect. The only times Christianity, for example, gets it right is when it bases its decisions on love. Morality from religious law is arbitrary and irrational. If it weren't then I would not be abhoring homophobia as much as you would be abhoring homosexuality, while both of us are deriving our theology from the same book.