Excuse me, umm, sir?

A few of my friends, clients and I were having a conversation about mistresses. Meaning the name a woman is called when she is a girlfriend of a married man.
We tried to figure out if there was a title for a man dating a married woman. When we could not find one we thought it best to make one up.

Some suggestions:
Man whore
Misteress
Masterer
My pick: Cockubine

Please add your own.