Im still in love with my ex boyfriend  though he's married now
That's something real considering the fact that an ex is someone with whom a person was once associated, in a relationship, marriage, or once talked to
Yeah!  But I still  love him, even if he's  my ex
Anyways, can refer to a variety of different relationships one might refer to a music group's "ex-guitarist
Ok, not only boy or girl friend  relationships 
It depends, if it is used as a noun and not a prefix yes. As a noun it refer to a former sexual or romantic partner
OK that's  quite thoughtful!  What other forms is it used? 
As a noun  it has a derogatory tinge, especially if it refers to unrequited feelings.
What do you mean by derogatory? 
Remembering that one loved a person and he didn't get loved back, this refers to unrequited feelings.