I waited until I was out of high school. I lost my virginity to someone special and then I had a ton of flings and one night stands, 95% of which were incredibly enjoyable and safe for everyone involved. Then I met my boyfriend, who was a 19 year old virgin. I've had sex with a couple and my boyfriend. I think this is completely acceptable for adults. However, not for teens. I think if teens have sex, it should be in a relationship with someone they care about.
"I want you to know that sex is a perfectly normal, natural thing. However, it is a very mature, messy, complicated thing. I waited until I was out of high school to have sex and I have never regretted it. My mom had me in high school and I didn't want that to happen to me.
"I do know people who had sex with long term significant others in high school and they didn't regret it. If you feel you love someone that much that you want to express your love for them in that way and you've been together long enough to trust each other, I will not judge you for that. That is your own decision and your right.
"If you make a mistake, I will not judge you for that. We all make mistakes. But you need to understand that meaningless, casual sex is not appropriate for a teenager. That doesn't mean it isn't something you shouldn't do or won't do as an adult. But as a teenager, you do not have a high school diploma and you are not ready to handle an unexpected pregnancy.
"I would like you to think really hard if you find yourself in a situation like that about whether or not it's something you would like to remember when you think back on it five years from then, or ten. I want you to think if it's worth it if something were to happen. I want you to understand that it has the potential to change your whole life.
"If you do have sex, you will need to use protection. <Insert futuristic birth control discussion here>"
"Understand that as an adult you will see sex as a completely different thing than you do now. You will be more apt to explore and do things that I would probably ground you for if I ever found out about you doing them now. You're an adult. You will be more in control of your emotions, more capable of handling that complicated, messy, weirdness that is sex. You'll have a diploma should you need to support yourself and someone else. You'll have more worldly experience and be more capable of making whatever decision you might need to make.
"I also want you to know you can talk to me about anything. I will try to understand and I will not judge you. You are my child and I love you and only want what is best for you. I know you might not make the decisions I would hope for you, but I would never love you any less for them."