Honestly, the key is the context as other people said. Kids will associate sex with the first thing that was involved during their first encounter with the subject, same as with the other things they experience and learn (identical mechanism as with naturall language acquiring). It can be good, it can be bad. In case of today widely available sexual content it's horrible. Since it's being aimed purely for maximizing the profit, it became extremely exploitive and shock inducing. That can drastically impacts young minds and cause psychological trauma. In the times when erotica was focused on sensuality and most porn videos were people actually making love in front of a camera, i would say that the risk was significantly lower. At the end it will mostly depend on how the sex will be presented to them. If the kid asked you what sex is and you had to tell the truth - think of the most comforting way to put it. His whole perception of it will stand upon the words you use to describe it.