Hey angelrose, Welcome to the Board.
You are not in a unusual situation. It's not uncommon for women, especially those that were married and started a family young, to begin to "find themselves" around 30. In fact most divorces in first marriages happen when the wife is between 28 and 31.
It's also not unusual to be attracted to other people besides your spouse. She has come to you with her feelings which says to me that she is a wonderful woman who loves and trusts you enough to tell you this and truly cares about your marriage. That also says allot about how good a husband you are!
The differences between marriages that work and those that end is how the members of the relationship deal with the above issues together.
The most important rules is swinging are: 1) if it doesn't work for either of you, then it doesn't work for both of you, and 2) you always go at the pace of the slowest one in the relationship.
Taking #1, if a situation makes either of you uncomfortable then the other one respects that and doesn't pursue it. This can be how you swing, who you swing with, what sex acts are off-limits while swinging, etc.
If her having sex with someone else makes you uneasy then it should not be done. I don't swing so Mrs. WS can have sex with others, WE swing because it is a huge turn-on for both of us to see the other having fun and getting their groove on.
We also wouldn't cheat on each other if we didn't swing. We can swing successfully because our heads are on straight like that. Swinging isn't "replacement sex" as so many "experts" in couples counseling would like you to believe. It is "additional sex" to an already great relationship and sex life.
If you are afraid she will cheat on you if you don't give her permission to have sex with this other guy then you two need to examine what's going on in your marriage that makes you think this way. Swinging is not a replacement for cheating. I don't cheat on Mrs. WS because we swing, I don't cheat on Mrs. WS out of love and respect for her and myself. I would never, ever want to hurt her in any way shape or form.
Taking #2, always going at the pace of the slowest person in the relationship. Even in swinging this changes. Sometimes it is me, sometimes it is Mrs. WS. In the beginning it was me so she let me drive this crazy train. Right now it is her.
What is huge red flags for me in your situation is 1) you're afraid if you don't let her play with this guy she will cheat on you, and 2) she won't afford you the same benefits and let you play with other women. This is very one-sided and tells me she's not interested in swinging, just having sex with this other guy. This is very selfish, and the very essence of swinging is that it is very unselfish.
So, in closing, you two need to really discuss why she is feeling this way, why she needs to have sex with this other guy, and how it makes both of you feel and how it will effect the marriage. You may also seek out some professional relationship help. Only swing, and I repeat, ONLY SWING when your relationship is on solid ground. Right now is not the right time.
Mr. WS