Few topics of parent/child interaction come with as much baggage as the conversations about sex. Usually, long before a parent is ready (and usually in a time/place not selected by the parent) the first questions come from a young child about pregnancy and babies. Children must learn from the very beginning that they are welcome and encouraged to come to you for information about any topic, especially sex.
1. Don't avoid the topic
If three-year-old Sally asks (loudly) in the grocery store line about the birds and the bees, you can predict a few grins from the surrounding adults. Simply stating to Sally (without an embarrassed reaction), that this is a topic you will discuss when you return home, is appropriate. Make sure you discuss it as soon as you return home, and give age-appropriate information. (Sally will undoubtedly pick up on the reaction from the other adults and you want her to be comfortable talking to you about sex now and 10 to 15 years from now.)
2. Don't disregard the effects of peers, the media, and the pervasive nature of pornography (internet, magazines and books) as possible influences to which your child has been exposed. With the expanding empire of internet pornography use (among younger children), many teens have been exposed to pornographic material. The images which they have seen will directly affect their view of sex, and need to be addressed. Discuss pornography with your child and the seriousness of developing a pornography addiction.
3. Don't give your child the third degree
When your child is dating, allow them their privacy. In other words, it is ok to discuss sexuality in general, but don't wait up and demand details from your child. It will further turn your child away from you and can push them further into sexual experimentation before they are ready. Show them that you trust them, and it will go a long way in the relationship.
4. Don't try and scare them away from sex
Not only do you want your child to avoid sexual behaviors before they are ready, you want them to develop a healthy view of sexuality that will stay with them into their adult life. Using scare tactics can actually give your child hang-ups about sex that will last far longer than the teenage years.
5. Don't try and use sexually transmitted diseases as a way to scare your teen
The threat of sexually transmitted diseases is real (especially HPV), and this information should be discussed with your teen. Remember that teenagers feel fairly invincible and their general attitude is that "It won't happen to me." They can engage in sexual behavior simply to rebel and prove you wrong on the STD issue.
6. Don't forget to listen to your teen and if you need to have a strong reaction, have it later. Let them speak openly with you, and without fear of judgment.
Finding out that a teen is already sexually active can be a blow to any parent. It is important to remember that your teen made these decisions, that they do not reflect on you and that your teen may need help to get through this road they have chosen.