Imagine that sex is swimming. Now imagine that the most powerful church in the world just happened to be founded on principles that extol the virtues of being landborne. Naturally, the church forbids the teaching of swimming. Their idea of "education" is describing the dangers of swimming and telling people not to enter the water. Over centuries, people learn to avoid swimming, and categorize it as a risky activity, mainly out of tradition (backed by the church's support).
As you can imagine, most people who dare to dive into the water unprepared end up drowning. And this just reinforces the church's position that swimming is dangerous. Meanwhile, the lucky few who manage to figure it out on their own have a real hard time teaching others, because the church actively tries to silence them. You see, it's in the church's interest to ensure that swimming remains dangerous, no matter how many lives are sacrificed for it.
Now, think about our world, where there is no powerful taboo against swimming. Swimming is still a risky activity, but the dangers are largely controllable. In fact, we have created man-made environments that allow for swimming while eliminating many of the peripheral dangers of being in the water. Most people know the basics of swimming; lessons often begin at a young age, and even before that, the inexperienced are permitted to splash in the shallows under proper supervision, and with the right protective measures in place. Some people still avoid the water, by choice, while others, who have an aptitude for it, seek out a deeper understanding of the aquatic environment, and even pursue careers in related fields. And none of this is cause for alarm, because it's all out in the open, where the risks can be monitored and reduced. And the lack of a taboo eliminates feelings of guilt and shame.
What, then, is the difference between sex and swimming? One is an activity without which most people could still live a fulfilling life, and the other is accompanied by a strong biological impetus, which when denied or repressed, leads to complex psychological problems.