You've got a friend...or not.

How long does it take to be able to acknowledge, when talking to an ex, that the two of you went out? Years? Decades? If you try to be friends with an ex, it just goes without saying that there will be a large blank period in the history of your "friendship," the events of which neither one of you will dare to mention to the other.
There's a long list of things that you're not supposed to bring up around someone you've been involved with. It includes any and all private jokes, all places where the two of you had sex or went on especially romantic dates, all songs that the two of you heard in intimate situations, and--most importantly--any statement that might have even the slightest sexual connotation. This last one is especially tough when you date musicians. You may think music is a safe thing to talk about with an ex, but all that fingering, blowing and shredding (well, OK, maybe not that last one, unless you're really kinky) can be a field of land mines just waiting to explode with awkwardness.
Given these parameters, it's not too surprising that more couples who break up don't stay friends. Unless, of course, you're one of those rare ex-couples, possibly existing only in movies, who give each other love advice based on mistakes they've made with each other. "OK. When you're making out with your new girlfriend, who is hotter, smarter and more sexually proficient than me, don't use as much tongue as you did with me." Riiiiight. Are you telling me that the average person, given this opportunity, wouldn't say, "Oh, no! You did everything perfect with me! Girls love a lot of tongue! Ever see parasitic fish using their mouths to stick to the glass at aquariums? Those bitches are hot!"