Hmm, well, sometimes I'm annoyed about that even if I know the person.
For example, for some time, my own activities of the sort were sort of non-existant due to some problem. Mr Kibbs (the only fitting nickname I've found for him so far
) was being nice enough to be, but things led to others and voilą.
Meanwhile, his best friend, Mister V, meets my cousin, Miss S, on a Tuesday and she goes to see him the following Friday night.
Saturday afternoon, they come pick me up after work (3:30 PM) since we're supposed to have dinner at Mom's place, where Miss S lives at the moment.
... they insist on going back by foot (15 minutes walk). During that walk, I was treated to their "breakthrough" of doing "things" for "quite some time" (aka all night). The moment they started this, I started moving faster than them. Miss S started insisting, so I literally ran (as much as I could, with my obesity and all) to Mom's, took off my coat, passed out on the couch and told Mom they are disgusting in some way.
They thought I was a fool for running ahead of them like that, but I mean, I love both of them (Miss S a bit less since things happened in the house, but still) but this isn't something to discuss like that, without warning, even with people you know really well. Especially since you've never talked about it before (Mister V said something too and I was horrified because, while he could make some jokes about the thing in general, it was mild up until that point).
I'm not that much of a prude, but sometimes, from some people, it annoys me to no end. Some people just don't care about who they have in front of them when they decide to talk, it seems u_u .