Wow there's so much I would like to say but don't want to come off as snarky.
First off tantra is not sex magick. Tantra is a word for spiritual teaching. There is a small part of SOME teachings that involve sex magick but not all. in fact, a large number of tantric practitioners are celibate because of the belief that men lose creative force and life through sex or even masturbation...
That being said I read one response that Pagans and Wiccans mostly practice sex magick symbolically. Some do, and I would even say that most Wiccans I'm aware of stick with symbolic sex magick only. But this is not to say most don't in fact I've read a few articles in the Llewellyn Magical Almanacs of the past 4 or 5 years that have had sex magick information.
If you're really interested in sex magick the works of Aleister Crowley and articles and books on Thelemic sex magick are recommended. There are also many modern books on sex magick available, do a simple search on Amazon for them.
I also want to point out that sex magick doesn't have to be involved with another person or poly relationships. You can create sex magick simply through masturbatory techniques.
I would certainly do more research though before I started trying this out if you're not sure of your religious beliefs or what path you would like to take.