Because kids are easily influenced. A kid that sees porn can think this is how actual sex, and relationship, work. However, pornography is a terrible model for that. Most of the time, men are pictured as powerful and women as submissive. Most of the time, the foreplay is stupid and unrealistic. Most of the time, the sex is pictured in a "passionate" way that looks really violent (thrusting like a madman trying to light a fire, slapping, pulling hair, spanking). Most of the time, actors don't use condoms. Most of the times, actors are above average when it comes to their bodies (big penises, big breasts, large hips, etc). Some times, they do insane fetishistic stuff. And so on. This can lead kids to think this stuff is normal when it's not. I'm not saying people shouldn't have fetishes or anything - I'm saying that a teenager is too young to decide if he likes BDSM or bukkake better. At that age, sexual education is of the utmost importance, and pornography is sexual misinformation. It's adding too many irrelevant things to the mix, and drowning the actually important and relevant information in fetishes, appearances, and dangerous behaviours. That's not counting the effects that can be associated with kids seeing genitals for the first time: traumas, questioning one's gender (not implying this is bad, but it can be painful), being scared of the other gender, etc. Porn can fuck up people.
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