If this guy had dumped you for your best friend or if it was a particularly bad break up where you can't stand the sight of each other I'd completely understand your point of view, but now I really don't - especially since it was you who refused him in the end. You can't 'reserve' a guy just in case your current relationship doesn't work out, and you shouldn't begrudge two of your friends happiness. I sincerely doubt you'd feel better if they'd 'asked your permission', so to speak; it sounds like you wouldn't have been cool with this however it started, and in a way it shows respect for your feelings that they didn't instantly tell you; they were probably trying to find out if it was going to work between them before telling you.
Honestly, I don't get this whole you can't date your friends' exes rule, especially not in a case like yours. My friends are welcome to any of my exes, although I'd strongly advise them to save themselves
Would you pay less for a humanities degree?