You've given some good descriptions here. But let me come at this from another perspective: A man's erection doesn't always mean sexual arousal. Any man or woman who really knows a man knows this can happen sometimes for "no reason at all," and most likely even happens to men while sleeping. (Even if they're not having "one of those dreams.") So the issue of men's erections has been inaccurately portrayed as sexual arousal. But then we can't always tell if this is spontaneous, or if "they're happy to see someone." In the end, erections are banned because it's better to be safe than sorry I guess.