There are further definitions, but for our purposes, we will focus on these. Sex magick may involve actually having sex, and maybe not. I recently did a class on the noncoital Great Rite, which does not have physical copulation as a part of it, but it can raise a vast amount of sexual energy. It is beneficial to know some of the techniques for noncoital sex magick, in case there ever comes a time when you are in a situation where the Great Rite is appropriate and your partner is not someone with whom you want to have actual sex. The fact that people have rather peculiar ideas about the whats, whys and wherefores of sex magick is not remarkable in itself.
Sex is taboo, and discussing the subject in company is almost worse than actually doing it.
Some people are going to have an adverse response when it comes to anything sensual or erotic. But much of the negative response I have received to sex magick seems to come from the fact that people think it has to do, somehow, with deception, manipulation or nonconsensual practices. You might use sex magick in that way, but I wouldn't recommend it, as it has almost instant karma. Sex magick is fairly well integrated into Eastern custom, with the Tao and Eastern thought maintaining that sex magick is a desirable practice. There seems to be much less shame, guilt and repression in the East, in some ways, and energy and energy work is a commonly accepted idea - think about Chinese acupuncture.