That's the problem. Personally I would say that if RM between Christians and Non-Christians were to become a thing, it needs to be something with a rule that is similar to: "Christians can only RM non-Christians, if one of the parties would be able to diplo-vassalize the other were they both of the same religion".There have been instances (especially earlier, from the Middle Ages) where royal marriages took place between Muslims and Christians, typically when the relationship between the two was highly unbalanced. Yet if it were made possible in the game it would be far more common than it ever was historically.