keep in mind that a fantasy sometimes is best left as a fantasy. when the things actually take place your and her reaction are often not what was expected to do. it can turn bad quick. if you want to go down this route it is best to test the waters in a way that doesn't show your complete hand.
first and foremost make it about her. if you only focus on how it affects you, she wont see any value in it. if she sees where it is only a win win for her, then there is something to talk about.
you have to maintain your image of masculinity the entire time. if she sees you as less masculine it will be a turn off for her, and she will lose respect for you. you need to maintain a level of control and self confidence the entire time. no jealousy ever.
start with mentioning how attractive she is. when you guys go out, point out men that were checking her out. even if they weren't. remind her on how much attention she gets, she is just blind to it. encourage her to dress sexy when you go out. when she does be sure to reward her with positive attention and affection. training that this behavior is what will get you positive feedback. keep working her confidence in this area. she needs to feel sexy and desired or nothing will ever happen.
When you tell her about some young guy or some alpha type figure checking her out, tell her that you like it. let it be known you are secure, not jealous, and love seeing her turn heads. even mention to her how you brag about her to friends and coworkers on how hot she is. or how they compliment you on what an amazing wife you scored. there needs to be a reminder she could have most any guy she wants.
if there is a celebrity she likes that is male, watch a movie with him in it with her. be sure to validate her attraction to him. say things like you can see why you like him. he is a charming guy, or he is handsome. I can see why girls like him. then be sure to mention things like, if you met him you could have him. she will probably say things like, "yeah right!" then laugh. but get the idea in her head about it. maybe even say something like, "if you guys ever met and he hit on you in front of me, I would let you guys do what you have to do. you would rock his world. he would start stalking you." this is a subtle hint that you are cool with certain men having sex with her, but only ones she choses.
after you can get this kind of thing going, bringing up that more normal guys that like her at the bar or club is also alright. that it actually excites you. but once again it needs to be all about her.
it can't feel ******, or that you are trying to set something up. baby steps. if she rejects it early you know not to continue. if you just start out with, "hey babe I want you to fuck other men." it won't go over well.
subtle hints here and there, setting up the mindset is key. she wont even consider it if it isn't real to her, and she sees no positive aspect. she will fear of everything she will lose before what she will gain. if she is comfortable, she will do it. you need to create comfort.