The argument gets used that because very young people engage in sexual activity the best thing to do is teach them about SAFE sex, rather than abstinence. It goes "you won't stop them doing it so help them avoid the risks." There could be flaws in this approach.
Firstly, society doesn't use the argument "because it happens, get used to it" about domestic violence or sexual assault. We keep trying to stop it from happening.
Secondly, it is a value judgement whether or not it's good that some children get involved in sex. Everyone is entitled to their own value judgement along with the rest of their opinions, but intellectual honesty requires you to admit that it IS a value judgement, not an absolute like the law of gravity. One arguement is that sexual experimentation perfectly natural; but then, so is aggressive behaviour, and we try to teach people to control that.
So there is a flaw in the arguement, sometimes rather smugly put, that to be realistic and sensible we have to teach children to use condoms, or whatever else. There is a valid place for telling people that abstinence may be best in certain situations. Peoples' moral views and beliefs are their own business; but it's ironic that libertarians object to Christians trying to impose their views on others while libertarians themselves insist they know what is best for every one else - and try to get everyone else doing things their way.
If people are encouraged to do just what they want, and to indulge any desire they have, then we see problems like fighting and violence because some insist on getting their own way and don't like being prevented. We get obesity and other diet problems because eating too much or eating junk food can be tempting. (I plead guilty!) "Experimenting" with certain substances can lead to some shocking disasters.
It can be really sad when what is meant to help people in fact hurts them. Did you ever hear someone say they wish they hadn't done something, but everyone else was so they did it too?
There can be a problem with saying that because a person wants to do something, that alone is reason to do it.
What is it that should guide life? Animal instincts or human conscience?