It's simple really - sex and lust are very basic emotions - almost animalistic. Now, human beings ARE animals, but we often don't like to tell ourselves this or accept it. I maintain we are the best animal, but we're still animals and we have animalistic thoughts and emotions. We can't exactly change that (until we figure out a way to alter the way our brains develop).
The denial of sex was seen as a way of making ourselves to be "more" than animals. We like to think that "we humans" don't do such things as have sex in the open or openly talk about it. It's seen as "primitive", "boorish" and "uncultured" to talk about sex all the time.
I was raised in such an environment. My parents were atheists, but any talk about nudity, sex or the opposite gender was strictly prohibitied. To this day, my family has an incredible urge to never, ever, ever mention even the word sex. I myself cannot express sexual attraction to women. I just can't. I can't do it. For me to talk about sex with my family would be like for someone else to talk about sex with their boss or grandmother.
Needless to say this has warped me considerably. I cannot express sexual attraction in real life. I never talk about sex and I do my best to show no interest in the subject. If I have to talk about it, I always use a clipped, clinical tone, like what a doctor would use. I never use slang when referring to women or any part of their body. I've tried to get a relationship going with quite a few women - never worked out because I couldn't bring myself to touch them or express sexual interest in them. Instead my "attempts" at courtship usually involve me debating them on topics, which usually ends with them hating me.
I'm fully aware of my problem - how much damage its done, how warped my personality has become. I blame my parents for that. However, I also thank my parents for giving me food and a good life, so it balances out.
Should sex be more openly discussed? Yes. But should it be more openly practiced? No - STDs are a real threat, and for some reason people are often too stupid to use contraceptives and protective devices.