2 issues I can see here...
1. Talk to your wife about it and if she says yes, do not absolutely under any circumstances allow her to meet someone without you being around and you need to set what is ok for you...only fucking, kissing is allowed or not,etc. you get the picture. If she meets with men without you there, it can lead to something you will likely not enjoy(her cheating or leaving you). Besides, this was something that you wanted so you need to be there. It can be exciting....
2. Limit this activity to once in a blue moon. Once a woman has a cock and the lover that comes with that cock, rock her world, you won"t be so appealing anymore..Take it from me who has not the eact experience but can tell you that even if you think you have a great sex life, opening that life to new and exciting experiences with other men can affect your sex life by makinf it seem lackluster or just plain not as good.
Shit, this answer was much longer than I anticipated. Just think it through since once she sleeps with someone else, the consequences can vary significantly on the experiences she is having.........
Good luck...I would love to be able to sleep with other men as part of my marriage and would respect the rules set by my husband. He isn't into sharing and I've had a taste of things I like better so after 16 years , I'm moving on...this isnt the only reason and I looked outside of my marriage because I have a really crappy marriage. Make sure that your marriage is strong enough to handle this......