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 became physically more attractive and carnally irresistible. The host gained an enormous sex drive, with limitless libido allowing them to engage in sexual acts for as long as they desire without strain.
The host developed special pheromones which were irresistible to others, they acted as an overpowering aphrodisiac capable of sexually arousing anything, naturally inducing pleasure and desire while subtly lifting inhibitions. The host was able to use the pheromones to easily induce the sensation of sexual orgasm in themselves or others at will. Mentally the host developed an instinctive mastery of seduction, sexual and carnal techniques for any partner.
However, when it inhabited Carys Fletcher, she would only willingly have sex with men.
Internally, the heart and brain would increase in size and the lungs shrink, which eventually would cause an explosion and kill the host. Owen Harper referred to the process to becoming "rat jam" after testing the effects to the gas on lab rats. (TV: Day One)
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.