Many Christians believe it is sinful, because generally they are uptight as hell about anything regarding sexuality.

I think the porn addiction thing is that you train your brain to seek the reward of sexual pleasure through the cheapest means possible: porn. Cheap as in fast, efficient, and often focusing on exactly what you want. Brain learns, “gee, this is a lot easier this way” and suddenly it’s more difficult to feel engaged with the more natural processes for sexy time.