sex is a LOOOOT less tabooed now than it was even, say, twenty years ago - but still pretty tabooed - in some circles. i have friends who discuss anything and everything openly as if they were talking about food.
as for the population that still does consider it taboo (moreso than suicide, violence etc.) is because of history, to be honest. most probably the victorian era, because last i checked, the bible talks openly about sex, pleasure, incest, etc. added onto that long line of tradition is the fact that most people have the ability to feel pleasure from sex. there's a guilt surrounding the fact that sex is pleasurable. violence? the majority agree it's wrong, terrible, makes you feel bad, so it's fine, it's in the open. the other examples mentioned are examples of what the majority of people view as something just plainly bad, and can all publically agree on. sex is like a weird secret humans have. hey, let's talk about this taboo thing which we secretly want but pretend we don't really know much about it or don't want to touch on.
or at least, that's my little view on it.
how freely one should talk about it should depend on the people they're with or the place they're in. it's better to be kind and considerate than someone who believes they're always right. sure you might have the right to talk about sex 24/7 but you're also a douchebag and creepy if you do that. so the goal is just being appropriate. some people don't feel comfortable talking about it because of bad experiences, etc. it's not nice to bring it up if someone doesn't feel good.
sex ed, right before or during puberty, as teen pregnancies are rampant these days. sexual consent i personally think is for population control and the fact that emotional maturity, financial stability, and the overall practical ability to raise a potential unwanted pregnancy does play a huge part in someone being able to make well-informed decisions. i've seen some 25 year olds make terrible decisions, but the chances of them being more financially stable and emotionally equipped to raise a child are of course, much higher.