There is a deep misunderstanding of the differences between Tantra and sex magick, primarily promulgated by a deep lack of knowledge about Tantra combined by humanity's eternal fascination with sexuality.
Tantra is not sex. It is not spiritualized sex. It is not an Eastern form of sex magick.
Tantra is a complete spiritual system, perhaps the oldest continually practiced form of Paganism in the world. There is no single thing that is Tantra, so claiming that "Tantra says," or "according to Tantra," is inherently misleading and false. In fact, there are numerous traditions of Tantra, many of which are not focused around sexuality at all.
As a complete spiritual system, a [i]part[/i] of Tantra does include a deep understanding of both physical and non-physical biology and physiology. Thus, in some Tantric traditions there is an understanding of sexuality that transcends most Western concepts of sexuality. But to say that Tantra is only about sexuality is like saying Wicca is only about the Great Rite.
That being said, there are Tantric traditions which focus on enlightenment through sexual practices. There are also methods of practical magick in Tantra which are usually focused on mantra (the power of sound), yantra (the power of images), and control of/surrender to the energies raised during sexual activity (and especially extended periods of sexual activity).
Thus, there are certain similarities between some Tantric sexual magickal practices and Western sex magick. However, the techniques are quite different, and some of the books that have been published show a total lack of understanding of Tantra and merely present Western sex magick with Sanskrit terms.
There's nothing wrong with that at all...but it's not Tantra.