Generally it's not a good idea to invite exes. Doing so has the potential to make the bride or groom feel uncomfortable; relatives and other guests might be distracted by it too. Talk to your fiancé about how you feel. Assure him that you are not opposed to his friendship with his ex but just don't want the reminder of their past relationship at your wedding. He should respect you wishes.
Note: If, on the other hand, you are all friends now and the relationship is decidedly water under the bridge, it is perfectly okay to invite an ex. The key is for both halves of the couple to be entirely on board with it, and for it not to distract guests from the focus of the day: your marriage.