Genuinely curious, as someone who was exposed to porn on several occasions as a kid (with seemingly no negative long-term consequences): what makes you think it's "really bad"?
There's a huge difference between regular exposure via neglectful or abusive parents and "oops I saw a thing". More than a few kids have "walked in" on their parents too, I assure you.
Seeing some girl getting ploughed by a slab of a guy might raise questions (which is excellent, questions are always good, because then you as a parent can guide their understanding), but it won't harm them in the slightest.
Repeated exposure to porn is a problem, but that comes from teenagers having an intrinsic interest in sex and private exploration of unsavoury websites on the internet, not accidental viewing of a naughty billboard.
Yikes.