I don't know much about sex magick, though you can reenact the act of sex without ever having sex. This is achieved by dipping a knife in a cup of liquid. These, of course, are sacred objects though, so it can't be just any old knife and cup. Also, sexual acts can raise energy within a circle, but the ones doing the acts must be focused on the energy and the ritual and magick they are doing, otherwise that energy is lost. Now about sex itself... That's another topic. It is beautiful, natural, okay to do outside of marriage, and it's okay to be sexual. There's a lot of controversy around sex, especially when it comes to female sexuality and such.