system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is it considered harming or disturbing for children to witness sexual content?

I don't get why we always have to "protect" children from seeing anything sexual in movies etc. 

Is there a psychological reason behind it or is it just a thing developed in our society ?
gpt
Because children are like little sponges that suck up every little bit of information they are exposed to, they also seek to emulate everything they perceive that grownups do. Finally they don't understand all the finer points of social etiquette, which is why they'll frequently do things like pick their noses or disrobe in public if you let them. 

If you let your kids be exposed to foul language or sex scenes, be prepared for many awkward situations that ensue when they start to emulate things they've witnessed or heard without understanding things like the context, consequences, social taboos and etiquette.

A simple example: your kid witnesses you give your SO a passionate kiss. Later that night when you tuck them in and give them a kiss goodnight, they try and jam their tongue down your throat. They have no idea why that would be inappropriate.