I try to keep such conversations with my son "casual". When we are watching television or after a movie, if I see a chance to discuss "seduction" I talk to him about the fact that as he grows up there will be women who will try to seduce him; some because he is attractive, some because they think sex is love, some because they want to get away from their parents and think forcing a marriage via pregnancy will do that. If a television show, news article or movie mentions ex-wives, child support, etc., I use it as an opportunity to discuss how young men can be stuck paying $$$$$ for the rest of their lives because of one night.
We talk about how he will be pressured to have sex by girls and by guys. The dangers of STD's, and the joys of knowing that one girl loved him enough to make him her only one.
I stress, over and over, that while I believe God prefers sex be in a marriage, that I will love him, no matter what, and I tell him, "Promise me, if you ever decide to engage in premarital sex, you will not trust a girl who tells you she can't get pregnant because she is on the pill. If she forgets it, even once, you could be a father for 60 years. Also, remember, condoms break, and STD's are things "no one ever gets" until suddenly they have given it to you."
I also try to gently encourage him toward Christian girls and Christian men. The parents all talk together, we all know each other. So, the general "peer pressure" with his group, at least for now, is "wait."
His older sister and her boyfriends also make a pointed effort around him to talk about how terribly easy for people to ruin their lives with sex.
I can't swear this will protect him forever, but it is a start.