I don't like the idea of calling authorities automatically when reporting a user for showing content on sexual assult children, on an online forum.

I don't have a problem with protecting children from sexual assault; but I see this rule on subreddits that if you post child porn, they'll ban and call authorities on you.

However, this system could be abused; an account could be falsely claimed of promoting sexual assault, then be falsely arrested.

I consider it more costly to punish falsely accused people; than to correctly punish criminals. (Relative, they both suck and the latter should usually and carefully be pursued.)