WHEN you are 15 and the whole world is in your future, it is hard to think about how the easy sexual pleasures of youth could kill you. But, remember, Magic Johnson (US basketball star who has been tested HIV-positive) was just 19 when he slam-dunked his way onto a professional basketball tour that gave him so many opportunities for easy pleasures. Teenagers today face more pressures than did their parents. Fitting in is not just a matter of how you dress or wear your hair, but whether you've ''done it'' yet. The problem is that early sexual experience increases the chances of getting a serious sexually transmitted disease, from herpes to genital warts to AIDS. And research also shows that, if you have any sexual disease, even a minor one, it will be easier for the AIDS virus to infect you if you encounter it in another sexual contact. So, being informed and responsible about your sexuality is a matter of protecting your own health. Abstinence really is the best way to avoid AIDS. But you still need to be informed about safe sex - because it is sometimes easier to believe in a rule than to live by it. One final word about drugs and alcohol, which, in a way, are part of the same issue. Let's be honest. Once you get drunk or high, your judgement is not great. And one more final word - about condoms: the more positive an attitude you have about condoms, the more likely you are to use them. In other words, if one of the reasons for having sex is that it makes you feel grown-up, there is nothing more grown-up than using a condom.