What I don't understand is that I can go to the cinema and watch the latest action film, and see potentially 100s of people getting killed and maimed (not to mention theft, speeding, etc, etc), and that's fine.

But if I want to watch something that's perfectly legal for 2 consenting 16 year olds to actually do in their own bedroom, that's illegal.

I understand a thing being illegal, but the depiction of the thing being legal. The opposite makes no sense.