Interesting discussion! I too have isues with the Christian/Catholic thing. Many assume it is the same thing, many during the Reformation were burned at the stake for thinking there was any difference.
Where the rules apply in today's world is what separates many from religion itself. If one believes, the rules are, well, the rules!
You don't change them because it is too hard to follow or because it is not popular. One can be tolerant of others though. My religions rules are my rules. What others do is their concern but I must do what I must do.
Englishgent has his rules and if family members want different rules that is their choice, but his choice has been made for him, IF he chooses to believe. It is a struggle in any religion, in the more strict it is even harder. The Orthodox Jews, the Amish, etc... have very strict beliefs, many directly from the bible as God's law, many from their own design in order to preserve the group. I'm not familiar with that many religions but will say that the Amish, the Mennonites and the Catholics are pretty strict too about marrying inside their own faith. Not just Orthodox Jews.
Even though the world is changing as Maurey pointed out many religions consider they are in this world, but not of this world and have other rules that must be obeyed above all else as part of their faith.
Tony