The "ex" prefix, used for everything from Presidents to girlfriends has one major ambiguity in its usage: what frame of reference, time wise, it applies to. For example, consider the following narrative:
"So there me and my ex-wife were, up on top of council crest, and even though we both are pretty conservative people, the romance got to use, and pretty soon we were rolling around right in the middle of the echo spot.
Now, even in this 21st century, people might look askance, or at least think it is curious, to be engaging in such activities with an ex-. But imagine the alternative...
"So there me and my wife were...and the romance got to us...pretty soon we were rolling around..."
This might cause confusion, as the person being addressed might think the speaker is still in such a relationship, which might cause confusion if, say, the person hearing the story is a cute cashier at K-Mart that the speaker wishes to woo.
In other words, the "ex-" term, when applied to things that happened in the past, doesn't make it clear whether the exness occurred contemporaneously with the events being described, or whether it refers to a current, that is, anachronistic perspective. One possible solution is to use the phrase "girlfriend (boyfriend/husband/dog walker/city councilperson) at the time", which while being descriptive, is somewhat clunky.
Until a better linguistic tool to correctly temporalize the state of exness exists, we will be surrounded by clunky explanations and/or misunderstandings. | 
