Suppose I tried to convince you that you should have a horn in the center of your forehead.
In order to convince you, I develop complex arguments, explaining all the values a horn would have. I draw analogies with the natural world, where horns can often be found, and often give the wearers advantage in battling adversaries and attracting partners. I point to art and culture, which cast the unicorn and its magical alicorn in a blessed and charmed light.
Since you're still not convinced, I dust off an ancient tome that I consider sacred. I find several verses of relevance. Some directly extol the virtues of horns, while others could merely be interpreted as pro-horn. Given that other lines lean so expressly in favour of horns, however, it strikes me that interpreting the ambiguous verses in a similar fashion just makes sense, really. I might also introduce you to a controversy among the behorned, the rift between those who favour just one horn and those who believe it is better to have more. While I clearly ally myself with the single-horned side, I note that even those who adhere to the Two-Horned Heresy at least see the importance of horns for humans. (There is also, I let you know, a small group whose members wear an artificial chocolate horn, but they have no real place in the debate. The Chocolate Heresy bars me from excessive pride; obviously, some in the pro-horn camp are silly, as are some among all human beings. Never mind).
But I see you are a person of science, so I put aside the grim fate that I'm certain awaits those who die unhorned. I instead appeal to science, or so I say. I note the number of surgical procedures that have successfully put horns on hornless heads. Several of the newly-horned claim in testimonials to feel so much better.
You are unmoved, and so I resort to mockery. I try outright taunts about your absent horn, trying to make you feel shame. I try subtle means, too, suggesting, that I thought you were "horning in" on a particular discussion, but then realized that couldn't possibly be the case, ha ha.
Would you be convinced? Would you arrange horn implantation? Would you at least feel some shame over your hornlessness?
I'm guessing not.
More likely, you would be baffled by my behavior. Annoyed. Alarmed, even. If I persisted in trying to convert you to the Way of the One True Horn, you would quickly consider me, at best, a nuisance who should be avoided or, at worst, a dangerous lunatic who should be restrained.
At the risk of being obnoxiously obvious: That's how many people feel when you try to convince them that your personal religious taboos are some kind of universal moral law. I'm not talking about murder or assault or theft here, things that clearly harm humans and human societies. I refer to things like, say, what speech constitutes blasphemy. Or what sort of sexual and personal relationships consenting adults should enter. If you want to follow such laws, be my guest. In most cases, I will fight for your right to do so. But you're not going to convince me you have the moral upper ground because you follow one or another ancient tribal custom. And if I've indicated I'm not open to further discussion on the matter, don't be surprised if, when you persist, or when you try to legislate your beliefs, I consider you, at best, a nuisance who should be avoided or, at worst, a dangerous lunatic who should be restrained.
Most likely, I'll look at you as if you have a horn in the center of your forehead.
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