An ex should obviously be someone you've had a bit more of a relationship with than "he who used to sit next to me on the bus." But don't make the cute li'l' guys stop using it, I love it when they say things about their "ex" who later turns out to have been their coloring partner.
On the subject of things to call a current "significant other" (that's a particularly stuffy one right there, actually
), if you don't like "my man" how about "my boo"? I think that's what you call 'em now for the three weeks before they become an ex.
I find all of those terms pretty annoying, but I do, in my head only, sometimes think of the bf as "my sweetie."
hee hee, even he would throw up if he heard that... I think that one is kinda cute. But I consider all those things to be too annoying for the world at large to hear.