The Ex is someone you most likely now Loathe, but at some point loved, or maybe just enjoyed having around, normally the result of a breaking up of a relationship of some sorts.
The word Ex is normally followed by these words..
Girlfriend, Boyfriend, Wife, Partner.
All these do apply..and a few more do too.
The easiest way to become an Ex, is to Cheat -
(mainly meaning to have sexual intercourse with another male/female whilst you are with you main boy/girl/partner etc)
BE WARNED! - your mates will hate you if you perform the above act, and they are friends with the guy/girl you cheat on.
Cheating will normally be accompanied by lies and deceit, in some cases a hidden guilt.