And it would be even more offensive, in my view, to have to listen to a discussion of the subject by someone who didn't think there was anything out of the ordinary in the fact that the discussion was occuring under those circumstances.
Would you be equally offended if you were in seat B, in the middle, and on sides A and C were two people discussing the bible?
I would be annoyed if I was in the middle seat and the people on either side of me were having a discussion about anything. That is incredibly rude. If this happened, I would offer to change seats with the passenger on the aisle. Assuming he or she agreed, I wouldn't care whether they then chose to discuss bibles or bimbos so long as they did it quietly.
Bottom line -- when others are not free to walk out of earshot, your right to free speech must be exercised within the limits of common sense and ordinary politeness.