But what they probably have not seen, yet, is real sex, respectful sex, the kind that comes naturally to people in love. That is why pornography is so harmful to the very young and why it was subject to an age restriction when that was possible.
Now parents and schools need to devise different ways of countering the distorted sexual messages young people are receiving so that their ability to form and maintain intimate relationships might not be permanently damaged.
The risk is not only that young men will mistake virtual sex for the real thing but that they will prefer it to the little reality they have experienced.
The task of parents and schools - and authors, film-makers, song writers and cultural leaders - is to enable young people to realise the richness of genuine relationships.
Film-makers in particular could do much better. Too often they depict sex as a casual diversion rather than a high point in a credibly developing attraction.
Schools are doing better on the evidence of the pupils who marched to Parliament. Those young people, male and female, clearly knew their age group was being fed a false and damaging distortion of sexuality on the internet.
At Parliament's gates a group of teenage boys handed out pamphlets for the White Ribbon campaign saying, "What kind of guy do you want to be? Yes to respectful relationships."
Yet not all schools are providing relationship discussion programmes, according to a researcher at Auckland University, Associate Professor Katie Fitzpatrick, who described the programmes in an article we published last Friday.
The Ministry of Education published a guide for sexuality education in 2015 but, she said, "There has been no meaningful professional development or support for teachers, and no extra resources to support this work."
Auckland Grammar introduced a programme last year. Principal Tim O'Connor has said, "If we can ensure they know the difference between an online environment and real relationships, then we're alerting them to what harm can be done."
All schools need to make the same effort. The young people demanding it at Parliament this week deserve it.