As the saying goes 'where there's a will, there's a bush' - people who are intent on having sex will find a way, even young people. Even young teens whose parents know where they are.
The vast majority of 13-16 yo don't have sex, but some do and some of those have happy, healthy relationships; many don't.
I get really quite irked by theses discussion sometimes because there is such a focus on 'sex' when really we should all worry much more about raising our children confident, with a strong sense of self-worth, boundaries, broad interests, hobbies they have fun doing and which give them a sense of pride.
I suspect that the vast majority of MN parents are involved in their children's lives, talk to them, worry about them and put a lot of effort in to their parenting. There are many children out there who do not have the privilege of growing up with loving support and many public policies are aimed at those very vulnerable youngsters.
I am appalled at the easy access to horrific violent hard-core pornography that v young teens now have - make the occasional underwear catalogue smuggled in to school by some of the boys in my class seem ludicrously tame by comparison.
I worry about what kind of world our children grow up in and the pressures they find themselves under (girls to be 'available' to all sorts of sexual activity, boys to 'want it' all the time). Knowing about the mechanics is the least of my worries tbh.
I think as parents we should not flatter ourselves that we can 'control' what our children get up to every minute of every day - no matter how well-meaning we may by. IMO the most important 'sex'/relationship education happens before they are 10: be kind to others and yourself, have respect and consideration and trust yourself - if you feel uncomfortable about something, it's probably wrong. The rest is up to them and us trusting their judgement. I have no idea what the next 10 years will bring for my offspring (eldest is 11), but all I am hoping is that I have encouraged open lines of communication. Fingers crossed.