In its natural form, the sex gas was a thick pink-purple gas with high concentrations of vorax and ceranium, which could move about at will. It could not survive in the atmosphere of Earth for very long and would quickly become a pink powder.
In order to survive, the gas would enter another organism through the mouth and nostrils. Whilst inside a human or a rat, the sex gas caused a variety of changes, both mental and physical. The host gained an enormous sex drive, sharing in the gas' need for the energy, creating an addiction. The host developed special pheromones which were irresistible to other members of the species of either gender. However, when it inhabited Carys Fletcher, she would only willingly have sex with a man.
The Sex Gas came to Earth in a meteorite that fell near Cardiff. It possessed a local female, Carys Fletcher, after being released accidentally by Gwen Cooper. On Earth, the gas found the energy produced during sex was the most beneficial (or so it thought). This sex was fatal for the other participants, who were disintegrated and absorbed at the climax.