Many parents do not even talk about sex with their kids. Embarrassment? Wishful thinking that their kids will never ever "do" it?
My philosophy is this: Safe Sex. I think that it's fine and dandy to proclaim abstinence as the safest way to go, but in this day & age, with our children inundated left, right and center by the various media, you would be living in a glass bubble if you think that they will not be tempted to experience sex especially in the 18 - 25 year old age group.
Emphasize responsible sexuality. Teach them that the act of sexual intercourse should be treated as something special which one experiences with a person when you are committed to each other. But having said that, we still need to make sure that our offspring are equipped with the knowledge to stay safe IF they do take that step.