So then you smile and say "I'm sorry, I chose this seat and I'm going to stay here" and move on. I'm really not getting the part where it becomes "how dare he have the nerve to think of asking me".
We're discussing a situation where a man states his request, and you say that the woman (who doesn't share his hashkafa) should just smile and either comply or not,and move on. You're surprised that the request itself would be perceived as rude by the woman.
Asking a question of a person (unlike asking Google) often provokes an emotional response. How do you think the woman might feel during this interaction? I would suggest that it would be similar to the way a man might feel if he's asked to switch seats because he is Jewish.
The hypothetical example of not sitting next to someone of a different religion is such a faulty analogy because inherently it's because of an animosity towards another religion, contamination from the non pure person, or a feeling of superiority towards that person.
You help me make my point. Although I made it clear that
The Christian in my analogy has nothing against Jews.
your gut response is that it must really be because he hates Jews, fears contamination, or feels superior. You're making these assumptions, based on experiences you've had or things you've learned in the past, not because of how he explained it to you. Just like the woman on the plane reacts to the request based on her prior experiences.
Both cases are similar in that someone is being asked to move because of the category of humans they fall into. It's considered rude to be told that someone wants to distance themselves from "your kind of people", whether it's women or Jews or any other group.
You've clearly described what's going through the mind of the man in this dialogue:
A pious male uncomfortable sitting next to a woman is ... just following what he believes to be a religious code of conduct not to come in very close proximity of an unrelated person of the opposite gender (see? I omitted "women" here) for a prolonged period of time.
But you don't seem to have given any thought to what might be going through the mind of the woman. There are two people in any dialogue.