Since everyone is made different, the 'accidentally slip' can and does happen. I dated someone and as much as you say the anus should be a tight bear trap, it just went in during sex, and it wasn't even me that did the accident. She was wet enough that it went right in as it slipped out and went into the other place. I actually stopped straight away, apologised and asked if she was OK.
I think it's like sticking your hand in a bag of skittles and expecting to get red every time, some people are just stupid and ignorant and expect to get a red skittle every time.
It will definitely be an interesting conversation wit our DDs, "Sweetie please remember it is never OK, unless you might have an aptitude for anal... do you?"
With the ease of media sharing these days, Amateur Porn and Cam Girls confirm that some people just have an aptitude for easy anal penetration. I don't think today it got normalised because of Porn, I think today it got normalised because of social media platforms... the same amount of people were probably doing anal in the past, but it was behind closed doors. Where would you even share that (Church, book club, bridge club, park?). I doubt men were too willing to tell their friends that their wives were into it out of stigma. I imagine it would be an interesting conversation to have with you mum or gran?
Saying that, everything that might cause distress should be assume off the menu until confirmed, or at least take it easy and see if the person allows you access. I imagine having a whole bunch of junk splurged in your eye, face, in your throat or any other place is just as violating if not discussed. Not to mention that some people could be allergic to sperm.