|1.||sex is on fire|
When someone is particularly attractive. Derived from the popular Kings of Leon song "Sex on Fire".
Damn, her sex is on fire.
|2.||Sex Is On Fire|
Like the icing on the cake, something happens to improve something else massively
see also sex is not on fire
-At this party tonight... remember I said Ben wasn't going?
-Well now he is
-Sex Is On FIRE!!!