As a parent I would tell them point blank that I know they've been watching porn, but that I'm not going to prohibit them from watching it (that will just shame their behaviour and suppress it). However, I would be very clear about some of the realities of the porn industry, such as how it can be exploitative of young women who need the money, same as prostitution.
I would also be clear about how porn is not representative of actual relationships and sex. That porn is usually overly misogynistic and degrading, and that it can set unrealistic expectations of what sex and relationships should actually be like.
Punishing and shaming them will only lead to other deep seeded issues, but they need to be educated.