This is EXACTLY the same thing I go through with my moms friends (and anyone else who knows me). They start talking about sex or something generally considered inappropriate and then they notice me and since everyone knows I'm a "pure little virgin girl" they assume I have virgin ears too. Then I crack a joke or join the conversation and you can pretty much see their brains explodes. I remember this one time my senior year of high school we were taking sex Ed (my second time taking the course thanks to transferring schools) and the teacher firmly threatened/promised that if anyone laughed they would get detention. Not even 5 minutes later she said something - I don't even remember what it was so long ago - and I started laughing. The entire class went dead silent and everyone stared at me like a herd of deer caught in massive headlights - even the teacher just stared at me awestruck. I asked why they were doing that and they all agreed that they didn't expect that from me of all people. I was the oldest I the class, the quietest, the teachers pet and the loner. Their reaction just made me laugh even harder to the point I cried. I told them that in my family you either get a dirty sense of humor or you don't survive. The teacher was so stunned she forgot to give me detention and I didn't ask her about it. Class went on normally and no one got detention that day.