Quoting Andi+Andy=Marley+1:" I never understood why women get upset when a friend hooks up or dates their ex. They are exes for a ... [snip!] ... then kick her out of the wedding. If you can suck it up, forgive her, and go on with your friendship then keep her in."
I wouldn't be upset if any of my friends dated my exes. But.....I never truly loved any of them. With my DH now, if my best friend hooked up with him...It would bother me. Not because of jealousy. But...because I would feel disrespected by her to sleep with someone who is the father of my child and someone I loved and spent years with.