A common problem many teachers have with teaching comprehensive sex ed is that they feel that it is not their place to talk to students about what our society considers such a morally controversial topic. Talking about sex is with adults is awkward for most youth, and so it is logical to conclude that it's likely not easy for adults to talk to youth about sex either.
One possible solution to the awkward problem is being implemented in several classrooms across Texas: computers. Evidence-based sex ed programs such as the one mentioned in the article below allow students access to the information they need while skipping the teacher as the middle man. Of course, teachers are available to answer specific questions, but I think most students and teachers would agree that it's much more comfortable to breach the topic with a machine.
http://www.ksat.com/news/23535302/detail.html
But it's scary to see that we're reduced to computers to explain good relationships to avoid controversy. And as important as abstinence is, it can't be the only thing we talk about. We have to give time to contraception, factual sti, and pregnancy information.