Unfortunately, I've found there are these stupid worms or viruses or whatever that "experts" with such crap, including "cyberporn experts," use to "piggyback" onto sites that use key words like "sex," in any form or context.
Usually they get filtered out, but popups & stuff can slip in sometimes. Regular virus scans & updates, watching the stuff Rax mentioned, helps a lot.
Unfortunately, when you get stuff popping up on your screen that gives no way to get rid of it, as the cybersexpushers love to do, sometimes even following the pattern Cement gave doesn't work. At least not for me on my computer. I know this becuz I've had it happen, and had to turn the computer off & restart it to get rid of the crap.
Of course that has only happened to me when I was bringing up porn in the 1st place, so there's an obvious answer to that particular problem. One that thankfully I've been able to put to work for some time now.
Muldoon, I'm sorry about the flashbacks. I haven't had any since around the 9-11 anniversary and few between then & the spring. But before that, back to THE 9-11, I had a flood of them. Sadly, they sometimes came when I was on recovery websites. Not always becuz of popups, but just becuz it could be triggering. Rare, thankfully.
I don't know if you've had flashbacks before or not. If so, you may know some ways to ease or work thru them. If not, feel free to message me here or PM if you want. Also a web keyword search should turn up something. The PTSD website Sidran has some good stuff on it.
Take care of yourself