Yes, jenn, the dialogue must be ongoing. You can't have a one time talk about how things work and expect everything's explained. As kids grow up, they pick all kinds of misinformation on the playground and locker room. The dialogue has to remain open.
Another thing. The talk to teens also needs a hefty dose of reality and possible consequences of their actions. Talk about various scenarios, 'heat of the moment' situations and how he/she is going to react, protection, methods of birth control and their correct use. Now, I'm sure someone's going to chime in and say talking about protection is condoning sex. Ask yourself this. Would you rather your teen become pregnant or develop an STD (HIV being only one of many nasty ones)?
You still can clearly state your views about early or pre-mairital sex. But, pardon the pun, sex happens. Make sure your child is prepared and ready to deal with it.
I'll get off my soapbox now. It's just that I feel very strongly about this. I've seen way too many unwanted, unplanned pregnancies and the concomitant fallout that occurs.