Sex has several meanings:
2. Sex refers to the expression of sexual stimulation and related sexual activities, e.g. fucking between two or more human beings of either gender sometimes happening spontaneously and sometimes after a nice flirt. While males can live without sex for long periods of time, women usually require sex at least twice a day (according to an unknown source :) ). Sex can be solo sex (masturbation), couple sex or group sex. For more information please check out the main article under sexual activity.
The most common use of the term "sex" is to express sexual stimulation: " I would like to have sex", "I need some sex tonight", "Is she going to have sex with me?", "When will we have sex?".