How would you react if you caught your 13 year old son watching porn? What if it was your daughter? (yes I know that's unlikley)
In either case I'd be okay with it but later explain to them that sex isn't really like that.
As long as they were just watching it and doing nothing else, I'd be perfectly fine with it. If not just watching it, I'll puke my guts out and not look at them the same way for a few days, then talk to them and tell them to only do that when mommy and daddy are hours away.