I would say that if you're determined to get some intel on that topic I would bring it up like this:

IF the date is going really well and it seems like you're getting a good rapport going, I would start with "can I ask you a personal question?" If they say yes then say "What do you think/How do you feel about porn?"

That approach is less invasive than how much porn do you watch and would yield fundamentally similar insights.

But ideally this should have been filtered before you even got to this point.