Rabbi Ahron Lopiansky said that discussing matters of a sexual nature in a public forum is improper. He said that if looking for a source, the Talmud in Shabbos 33a was pertinent. He never acknowledged this as a definitive source, though:
Said R. Hanan b. Rabbah: All know for what purpose a bride enters the bridal canopy, yet against whomsoever who speaks obscenely [thereof], even if a sentence of seventy years' happiness had been sealed for him, it is reversed for evil.
Alex's accepted answer to this question about tzniut, modesty, mentions the idea that what is private should remain private, citing biblical verses, and the Talmud.
I think the delicate nature of certain topics, namely sexuality, is evidenced further in Nedarim 20a-b, where Ima Shalom describes the reverent practices of her husband, R' Eliezer, during marital relations, as a reason for her children's beauty. A saying of R' Yohanan in Sukkah 52b bears mentioning as well:
"A person has a small organ. If he starves it, it is satisfied. If he feeds it, it is hungry"
The idea seems to be that approach to a person's sexuality a dangerous thing, because indulgence only increases, rather than sating, one's desire. So one must be very careful when it comes to this area.
Rabbi Lopiansky has dismissed the following as the wrong source, as this pertains specfically to matters of kiddushin, jewish marriage, and not to specific activities in a non-forbidden context:
The Mishnah on Chagiga 11b, (cited by Double AA in the comments on the question), states that one may not expound upon forbidden sexual relationships before three pupils. Rashi explains that this refers to forbidden sexual relationships that are not clearly mentioned in the verses, but are derived from them. He cites Sanhedrin 75a.
In the gemarah, the reason given is that while the rabbi is speaking with one of the students, the other two students will be able to talk amongst themselves, rather than listen to the rabbi, and due to the underlying bias of perpetual sexual desire, they may arrive at the permission of something that is prohibited.
In a broader sense, it would seem that in the absence of a rabbinic authority, discussing topics of sexuality may lead to forbidden sexual activity.