How about filling out your profile and then giving the forum an overview of your relationship with your fiance'. Have the 2 of you ever included others when having sex? What kind of fantasies does she have? ect ...
If the 2 of you have never done anything, other than with each other, she's not going to cuckold you Wannabe. Your best bet would be to do some fantasy play, possibly include a 3rd person at some point, THEN she might feel it is ok, but right now she's probably focused on getting married and she's not going to screw that up.