What are the Jewish customs on sex education of children or teens? How is it done, and at what age?
Or is it completely avoided until marriage?
Are they taught what it means for example a man can not sleep with a man, Yehuda and Tamar...?
Books or sources on the subject?
I know of two books which address the topic with sensitivity and respect for halacha
Let me summarize is in short:
The general Haredi perception of the purpose of the intimate relations is [still] procreation. Lately, especially abroad (out of Israel) it is slowly changed into a necessary "leisure" part of Shlom Bays. I'll mention this later on.
The approach to sex education is similar to other Mitzvos that are not studied or discussed until applied. Should I learn how to slaughter? I won't if it's not practical for me, so why learn relations without relations? Also Hilhos Ishus, Niddah etc. are only learned when applicable (for married men).
Besides the simple "closeness" (והאדם ידע את אשתו) between a man and a woman mentioned in the sources (Mikrah, Gemmorah etc), no details are discussed until marriage. Actually, this is pretty smart, because it is not only a waste of time but very impractical (not mentioning לא תתורו which is a clear prohibition). It provokes unnecessary curiosity and possible very serious transgressions of ז"ל, attempts of gaining personal experiences etc.
Sexual sins (ז"ל, משכב זכר, עריות) are freely mentioned within the learned material thru the Yeshivah years, but (as being pretty much self-explainable) never studied in details (like what exactly constitutes the עבירה and the likes).
[only] Prior to the wedding, the bride and the groom take a [short] private course with Madrich Chatanim/Kalot that explains what's done and how it's done. For most guys and [less for] girls it is a huge psychological shock, so usually it takes months up to a year to figure out the right intimacy (many frum guys can't function at all in the beginning), therefore this guidance continues into their marriage.
Lately, the awareness of women's needs grows even in Israeli Haredi community and the groom is instructed not only on the ways of mere procreating but also pleasuring his wife [not necessarily naturally] as I brought in #1. This leads to more and more Madrichim (and Rabbis) awareness and recommendations for the guys on using accessories for women (some Chassidic groups have their own private shops(!), but you need Rebbes approval to visit those).
Needless to say, as the access to media that lacked for thousands of years is becoming pretty ubiquitous, many of today's Yeshivah students have Internet and all the information they need in verbal and graphic forms. This situation is similar to what happens in the secular world but a couple of years later. This is a huge problem for the whole system and especially Rabbonim, but it is natural and needs to be dealt with.
Another issue is contraceptives. The Halachic standpoint is pretty clear hear, so it is practically only the "women's problem". However, as I stated, the awareness for non-procreative relations is going strong and it slowly becomes a standard with Haredi women also, not only as an excuse as it was perceived for years.
