An ex-lover with whom you have sex.
I had ex sex with my sex ex the other day.
A person (male or female) that you used to date (but no longer do), but still will sleep with you now and again.
See also Imaginary
Ex: "Hey, I know we stopped dating because you're tremendously creepy, but I think we can at least be sex exes, don't you?"
Other person: (No response; tends not to talk to people he's imagining in his head)