The "cancer" would be psychological. There is zero chance, no matter how universal, that we could possibly get unquestionable evidence about that. People still don't believe in ADHD. what makes you think we'd have a snowballs chance in hell that everyone would see mental issues and just nod their head and say "lets ban porn".

We don't even have unquestionable evidence about smoking. Tons of people say things like "I'm more likely to be hit by a car so I'm just going to smoke"