Opening a dialog.

 One of the down falls of the sex education programs being used in so many school is the vocabulary.
 Like any topic you want to discuss it really helps if you are both speaking the same language. Unfortunately kids are taught from a very early age that some body parts we do not talk about. So even if they are familiar with words like vagina, labia, aureola and penis they may be way to embarrassed to ever use them in a dialog about sex. 

Instead the vocabulary they use is one they get from their peers and from the media.
Songs especially are full of lyrical lines about sex, but not the way you might think.
An example is Chris Brown's "Sex" Where he sings explicitly about rough sex and sex without consent.
Beyonce also has sexually explicit lyrics.
Just to name two of the tamest.
If you ask your child what music they listen to, chances are they will be listening to something involving sex even if they assure you there are no "bad words" in what they are listening to.

But songs are not the only avenue kids have to learn about sex, and love and commitment and relationships. There are lots of videos, games, stories available on the internet free for the asking. A stern warning about adult content is hardly going to form any kind of deterrent to a curious mind. Too often what they witness may be far from the education about sex you thought they were getting.

So get familiar with the vocabulary your kids know and then help them learn yours.
If you sit down and think of all the words for breasts and buttocks and penises it no wonder it can all seem very confusing. And that is just the anatomy part of the discussion. When it comes to physical sensations, emotions, respect, consent and all the enjoyable things about sex the language can get really confusing.
Laci Green puts out videos about sex that are basic, open and enthusiastic if you want an example of someone who has the vocabulary and makes connections in a positive way.
https://www.youtube.com/user/lacigreen