I think it is unrealistic. That doesn't mean that I think they are all out there "doing it." I agree with RobinRS, a lot of it has to do with the kid's personality.
I am embarrassed to say that I did not abstain as a teenager. I am lucky that nothing happened to me that was bad. But I came from a very good home, my mom was a virgin when she got married, didn't smoke or drink, didn't have boyfriends over, etc. She was appropriately strict with me, but not over the top. We had a great relationship and everything was as it should be. But I still did it. And it had absolutely no bearing on what my parents did or did not do. My best friend, on the other hand, came from an unstable home. Her mom was never there, was married three times, had various boyfriends come to the house, partied all the time. My best friend didn't have sex until she was ready to marry the guy. She said she just never felt that pressed to do it and she felt that her hormones were not really "raging."
So, I think that parents should expect that it might happen. It doesn't mean you don't trust your teens, it just means that you are aware that the possibility exists, no matter how "good" they are.