Not everyone does though. My mom is one of the most prude person I know, to the point I wonder how she managed to ever have three children. Every time we talk about sex, or make sex jokes with my brothers and father, she blushes so much and asks us how we can even dare to talk like that. We laugh it off, but it makes her so uncomfortable, it's cute. I've had many friends like that as well, and I'm always a bit sad when people are SO shy about sex. I wished it wasn't SUCH a taboo subject, it should never be, I think it's unhealthy as hell when it is.
Fine, hold onto your cultural anomalies. 
My family is mostly hardcore Lutheran. I am a debauched heathen. 
They vary between making agonized faces at an actually rather subtle sex joke by my cousin's girlfriend at a gathering, or making the younglings eyes bug out.
I maintain this proud tradition of flip-flopping from sentence to sentence how comfortable I am with it. 
I could bring up the political and artistic implications of pornography from Pompeii in a class and discuss it avidly without being phased, but my mother asking if I had had my first kiss yet with my SO was an act of war. 
OT Trivia of the day: There are tribes in Polynesia, I believe it was, were the men wear heavy make-up and will run from a battle if it starts to rain, and the ideal women is considered to physically resemble a pig as much as possible.