One of the things I like to do at the start of this topic is to ask students to call out words or phrases they know that are associated with sex and sexual reproduction, and write them on the board. The aim is to try and get the inevitable giggles and snickering at least partly out of their systems! This is also quite a fun activity to begin the topic, and hopefully lets my students feel that they are free to raise any topics or ask any questions that they may be curious about.
As I was building up their list of words on the board, one student jokingly proposed 'inappropriate topic' as her phrase. I asked what she meant and she said that the activity was appropriate 'scientifically' but not for a classroom of 13-year olds. This raised an interesting discussion point, and led me to explain that research suggests that earlier sex education does not, as some would have it, lead to kids having sex earlier, but that when they do start becoming sexually active they do so in such a way that STIs and unwanted pregnancies are less likely to be the outcome. Of course, the topics and content of sex education should be age appropriate, and there are some great resources for age appropriate curriculum materials to be found here.
I can't speak for other schools in Thailand, but my school at least is fairly progressive in terms of sex education, and it crops up in both the science and the health curriculum. In the past my health teacher colleague has invited guest speakers from a local NGO, The Population and Community Development Association (PDA), to talk to students about various aspects of sexual health. (The PDA are also known for their popular, sexual health awareness-raising Thai restaurant, Cabbages and Condoms).
For me personally, I think it is both irresponsible and negligent to not give our students the best possible advice, with age appropriate content, when it comes to education around sex, sexuality, and sexual health.