If they watch it too much and masturbate too much, their sex performances will drop. Any addictive habit can affect ones life if it gets too bad.
In line with the conspiracy it's that increased watching of porn causes unrealistic expectations which leads to ED and decreased performance or creates fetishes that cannot be achieved with the opposite gender so if you're not having it it doesn't matter if you want it or not. To call it conspiracy directed at white people or a conspiracy with any goal besides appeasing peoples increasingly far out fetishes in the pursuit of profit is ridiculous though. I do think porn is unhealthy after a certain point and leads to unrealistic expectations and ruins connection and intimacy between people. Marc Maron has a funny bit about it.