First, you need to do the whole birds and the bees talk. You can't run before you can walk, and if a child hasn't received a suitable grounding in this then the rest will be a real problem to distinguish.
On the asumption that the purely biological process is understood by the child (not something I have done yet with mine, but that's a whole other question), then you need to move onto porn from there (assuming your child is of a suitable age that they have access to and understanding of how to use the internet and so on).
To answer your question from this point, I would say that you need to distinguish the act of lovemaking between 2 adults as a private activity between a couple, and the public activity of being filmed whilst engaging in it. Make it clear that any films that they might see on the internet fall into the latter category, point out that in addition to the people being filmed there are more people sat in the same room actually doing the filming, often more than one person, so while it may look like you are watching two people engaging in a private act, you are not really, so it's not real life.
For a child friendly analagy, it's like going to the toilet: there are some things you don't share with other people, and certainly not strangers. Anyone who does share pictures or film of them going to the toilet is not really being very nice, right? Same with people having sex, you keep it between you and the other person, anyone who doesn't is just being weird too, it's equally not nice and equally shouldn't be shared.
Hopefully you can convince your child that the people why might see doing this are not necessarily nice people, and try to turn them off their thinking it's titillating by making them think it's weird instead.
Obviously an adult could pick holes in this, but hopefully a child won't.