Or worst of all, a relative? What was your reaction?
I haven’t …yet.
Though it’s not from a lack of trying. 
We did this 3 months ago: ..surfing porn... WHOA! I KNOW THAT PERSON! - Miscellaneous and Personal Stuff I Must Share - Straight Dope Message Board
To be fair, the subject line of that thread was annoyingly vague. It could’ve been about seeing someone at the supermarket for all we knew. It wouldn’t have hurt to say it was about porn seeing as the text was specifically about it.
I can’t remember if I answered in the other thread but the answer is yes - I’ve seen about five people in pornos that I know (but then I did move in those circles for a while). One of them I’ve actually had sex with whilst watching said porno; that was fun.
I saw my old roommate, from Mississippi, streaming on the big TV in a sex shop in Berlin. That was…weird.