Wow. What an interesting thread.
I am a Christian.
I am a parent of teenagers.
And, I am a teacher of middle schoolers.
I think most previous posts made very good points, however, I tend to agree most with jillybean...teach the facts. The fact is that having sex can mess with you physically as well as emotionally.
The responsibility of teaching is the responsibility of the parent - not the school. However, look around and you will see there are many, many parents not doing their job. I actually had a student ask, rather comment, on the fact that after her Quinceanera it was okay to have sex. She would be ready then. It was at that moment that I realized that our kids were not necessarily irresponsible (although many are!) but many are simply uninformed.
The schools MUST step in and help. Did I talk to my kids about sex? Of course.. Not a sit-down birds and bees lesson but I gave them what they could handle at the time. Constantly. I don't try to overload them and try to make them feel like they could come to me with whatever question they had (and, boy, did they!) Do I appreciate the school backing me up? Of course. If I didn't want them to go through the school program I would simply opt out but, honestly, they need to hear it from someone else, too. Do I feel like I have taught them from a high-moral opionion? Of course...but, I am Christian not an idiot. I made mistakes and my kids will make mistakes. My prayer here is that my kids don't make the same ones...

Especially when it comes to sex. That is something that will never go away. Never.
Incidently, the middle school that I teach at has at least one student pregnant every year. A 7th grader just had a baby a month ago...The problem in my community is not necessarily the 'safe sex' talk, but that they don't do it soon enough! (puberty in 5th, sex in 7th and 9th...sigh)
How sad is that....?