I think it depends on how "aX" feels about it.
There's some ex-boyfriends I wouldn't care if my sisters dated. Well, I'd worry about their sanity, but I wouldn't be jealous. And then there's those boyfriends who are either scum of the earth that I wouldn't want any poor girl to date, or who I was extremely serious with and it would be really weird if they dated them. But most of the guys are grouped in the first category. Now that I'm in a very serious relationship, I realize that all significant others have been, well, insignificant.
Basically, I'd say it's up to bX and aX. aX needs to figure out how she feels about it, and then bX needs to decide if she wants to pursue it anyway if aX isn't comfortable with it. If aX has dated another guy for awhile, she might be cool with it. But if you two were only an item for like a month, then hopefully she's over it.
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