The book "Sexual Attitudes and Lifestyles", which reports one of the first detailed sexual behaviour studies ever carried out, certainly in this country, and which came out in 1994 following concern about HIV, found that for women aged 55 to 59 (at that time), fewer than 1% had experienced sexual intercourse before the age of 16, compared to 18.7% of women (almost 1 in 5) aged 16 to 19. The median age for age at first intercourse for those aged 16 to 24 was 17 years (so half the population had sex earlier and half had it later). No doubt, 17 years on, there are now more than one in 5 girls who have sex before 16, perhaps it is even up to 2 in 5. If so, out of 100 girls aged 15 and 3/4, perhaps 40 of them will have had sex (though they may not be having sex regularly).
So we are faced with a problem that some girls (and probably a substantial number/proportion), for whatever reasons, are going to be having sex...and in my view, if they are going to have sex anyway, it is important that they have the information they need in order to do it as safely as possible. At the same time, you do want to discourage them from doing it--the survey detailed in the book quoted above found that almost 60% of women who had first sex before 16 thought that this had been too soon. (The equivalent figure for men who had first sex aged under 16 was almost 25%.)
It is a tricky one. My strategy has been to have regular discussions over the dinner table with our DDs (17, 15 and 13), often prompted by stories in the paper or TV progs we have watched together. All three go to a mixed comprehensive and see boys every day in all their glory. They have seen friends who go to girls' schools literally jump on some of the boys they know, because these girls are simply desperate for contact with the opposite sex. I actually think that going to a mixed comp has worked very well in holding my older two back from early relationships of this kind. (Not sure I can say the same for the youngest one, whosome on this thread will knowhas a boyfriend and I have had to stop her going round to his place with no adults present. That is working well at present.)
Actually, clearly it is also important to stop or delay early sexual experience (as opposed to early sexual intercourse), because the same book reports that "the interval between first experience and first intercourse seems to be diminishing over time, more markedly for women than men": for women aged 45 to 59, the average time lapse between the two was 4 years or more (2 years longer than for women aged 16 to 24).
It is true that if a girl has decided she is going to have sex, there is little that will stop her other than "locking up your daughters". I speak from personal experience--what was I thinking of, it was a miracle I didn't come to some harm. It is our knowledge as parents (my DP also had a similar young experience though no risk of pregnancy for him) of what can go wrong at that age (partly as a result of inadequate or absent parenting), that has made us take a firm line with our girls about the risks and responsibilities that come with having sex, and that they need to be sure they can cope with these before they start a relationship of that kind.